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Commit e5c68284 authored by Peter Zijlstra's avatar Peter Zijlstra
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futex: Split out requeue 

Move all the requeue bits into their own file.
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarAndré Almeida <andrealmeid@collabora.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarAndré Almeida <andrealmeid@collabora.com>
Link: https://lore.kernel.org/r/20210923171111.300673-14-andrealmeid@collabora.com
parent 95c336a7
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kernel/futex/Makefile
View file @ e5c68284
# SPDX-License-Identifier: GPL-2.0
obj-y += core.o syscalls.o pi.o
obj-y += core.o syscalls.o pi.o requeue.o
kernel/futex/core.c
View file @ e5c68284
......@@ -147,64 +147,6 @@ int __read_mostly futex_cmpxchg_enabled;
#endif
/*
* On PREEMPT_RT, the hash bucket lock is a 'sleeping' spinlock with an
* underlying rtmutex. The task which is about to be requeued could have
* just woken up (timeout, signal). After the wake up the task has to
* acquire hash bucket lock, which is held by the requeue code. As a task
* can only be blocked on _ONE_ rtmutex at a time, the proxy lock blocking
* and the hash bucket lock blocking would collide and corrupt state.
*
* On !PREEMPT_RT this is not a problem and everything could be serialized
* on hash bucket lock, but aside of having the benefit of common code,
* this allows to avoid doing the requeue when the task is already on the
* way out and taking the hash bucket lock of the original uaddr1 when the
* requeue has been completed.
*
* The following state transitions are valid:
*
* On the waiter side:
* Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_IGNORE
* Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_WAIT
*
* On the requeue side:
* Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_INPROGRESS
* Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_DONE/LOCKED
* Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_NONE (requeue failed)
* Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_DONE/LOCKED
* Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_IGNORE (requeue failed)
*
* The requeue side ignores a waiter with state Q_REQUEUE_PI_IGNORE as this
* signals that the waiter is already on the way out. It also means that
* the waiter is still on the 'wait' futex, i.e. uaddr1.
*
* The waiter side signals early wakeup to the requeue side either through
* setting state to Q_REQUEUE_PI_IGNORE or to Q_REQUEUE_PI_WAIT depending
* on the current state. In case of Q_REQUEUE_PI_IGNORE it can immediately
* proceed to take the hash bucket lock of uaddr1. If it set state to WAIT,
* which means the wakeup is interleaving with a requeue in progress it has
* to wait for the requeue side to change the state. Either to DONE/LOCKED
* or to IGNORE. DONE/LOCKED means the waiter q is now on the uaddr2 futex
* and either blocked (DONE) or has acquired it (LOCKED). IGNORE is set by
* the requeue side when the requeue attempt failed via deadlock detection
* and therefore the waiter q is still on the uaddr1 futex.
*/
enum {
Q_REQUEUE_PI_NONE = 0,
Q_REQUEUE_PI_IGNORE,
Q_REQUEUE_PI_IN_PROGRESS,
Q_REQUEUE_PI_WAIT,
Q_REQUEUE_PI_DONE,
Q_REQUEUE_PI_LOCKED,
};
const struct futex_q futex_q_init = {
/* list gets initialized in futex_queue()*/
.key = FUTEX_KEY_INIT,
.bitset = FUTEX_BITSET_MATCH_ANY,
.requeue_state = ATOMIC_INIT(Q_REQUEUE_PI_NONE),
};
/*
* The base of the bucket array and its size are always used together
* (after initialization only in futex_hash()), so ensure that they
......@@ -269,31 +211,6 @@ late_initcall(fail_futex_debugfs);
#endif /* CONFIG_FAIL_FUTEX */
/*
* Reflects a new waiter being added to the waitqueue.
*/
static inline void futex_hb_waiters_inc(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
atomic_inc(&hb->waiters);
/*
* Full barrier (A), see the ordering comment above.
*/
smp_mb__after_atomic();
#endif
}
/*
* Reflects a waiter being removed from the waitqueue by wakeup
* paths.
*/
static inline void futex_hb_waiters_dec(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
atomic_dec(&hb->waiters);
#endif
}
static inline int futex_hb_waiters_pending(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
......@@ -323,21 +240,6 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
}
/**
* futex_match - Check whether two futex keys are equal
* @key1: Pointer to key1
* @key2: Pointer to key2
*
* Return 1 if two futex_keys are equal, 0 otherwise.
*/
static inline int futex_match(union futex_key *key1, union futex_key *key2)
{
return (key1 && key2
&& key1->both.word == key2->both.word
&& key1->both.ptr == key2->both.ptr
&& key1->both.offset == key2->both.offset);
}
/**
* futex_setup_timer - set up the sleeping hrtimer.
* @time: ptr to the given timeout value
......@@ -713,7 +615,7 @@ void wait_for_owner_exiting(int ret, struct task_struct *exiting)
*
* The q->lock_ptr must not be NULL and must be held by the caller.
*/
static void __futex_unqueue(struct futex_q *q)
void __futex_unqueue(struct futex_q *q)
{
struct futex_hash_bucket *hb;
......@@ -732,7 +634,7 @@ static void __futex_unqueue(struct futex_q *q)
* must ensure to later call wake_up_q() for the actual
* wakeups to occur.
*/
static void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
{
struct task_struct *p = q->task;
......@@ -757,30 +659,6 @@ static void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
wake_q_add_safe(wake_q, p);
}
/*
* Express the locking dependencies for lockdep:
*/
static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
if (hb1 <= hb2) {
spin_lock(&hb1->lock);
if (hb1 < hb2)
spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
} else { /* hb1 > hb2 */
spin_lock(&hb2->lock);
spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
}
}
static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
spin_unlock(&hb1->lock);
if (hb1 != hb2)
spin_unlock(&hb2->lock);
}
/*
* Wake up waiters matching bitset queued on this futex (uaddr).
*/
......@@ -961,619 +839,6 @@ int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
return ret;
}
/**
* requeue_futex() - Requeue a futex_q from one hb to another
* @q: the futex_q to requeue
* @hb1: the source hash_bucket
* @hb2: the target hash_bucket
* @key2: the new key for the requeued futex_q
*/
static inline
void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2, union futex_key *key2)
{
/*
* If key1 and key2 hash to the same bucket, no need to
* requeue.
*/
if (likely(&hb1->chain != &hb2->chain)) {
plist_del(&q->list, &hb1->chain);
futex_hb_waiters_dec(hb1);
futex_hb_waiters_inc(hb2);
plist_add(&q->list, &hb2->chain);
q->lock_ptr = &hb2->lock;
}
q->key = *key2;
}
static inline bool futex_requeue_pi_prepare(struct futex_q *q,
struct futex_pi_state *pi_state)
{
int old, new;
/*
* Set state to Q_REQUEUE_PI_IN_PROGRESS unless an early wakeup has
* already set Q_REQUEUE_PI_IGNORE to signal that requeue should
* ignore the waiter.
*/
old = atomic_read_acquire(&q->requeue_state);
do {
if (old == Q_REQUEUE_PI_IGNORE)
return false;
/*
* futex_proxy_trylock_atomic() might have set it to
* IN_PROGRESS and a interleaved early wake to WAIT.
*
* It was considered to have an extra state for that
* trylock, but that would just add more conditionals
* all over the place for a dubious value.
*/
if (old != Q_REQUEUE_PI_NONE)
break;
new = Q_REQUEUE_PI_IN_PROGRESS;
} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
q->pi_state = pi_state;
return true;
}
static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
{
int old, new;
old = atomic_read_acquire(&q->requeue_state);
do {
if (old == Q_REQUEUE_PI_IGNORE)
return;
if (locked >= 0) {
/* Requeue succeeded. Set DONE or LOCKED */
WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
old != Q_REQUEUE_PI_WAIT);
new = Q_REQUEUE_PI_DONE + locked;
} else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
/* Deadlock, no early wakeup interleave */
new = Q_REQUEUE_PI_NONE;
} else {
/* Deadlock, early wakeup interleave. */
WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);
new = Q_REQUEUE_PI_IGNORE;
}
} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
#ifdef CONFIG_PREEMPT_RT
/* If the waiter interleaved with the requeue let it know */
if (unlikely(old == Q_REQUEUE_PI_WAIT))
rcuwait_wake_up(&q->requeue_wait);
#endif
}
static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
{
int old, new;
old = atomic_read_acquire(&q->requeue_state);
do {
/* Is requeue done already? */
if (old >= Q_REQUEUE_PI_DONE)
return old;
/*
* If not done, then tell the requeue code to either ignore
* the waiter or to wake it up once the requeue is done.
*/
new = Q_REQUEUE_PI_WAIT;
if (old == Q_REQUEUE_PI_NONE)
new = Q_REQUEUE_PI_IGNORE;
} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
/* If the requeue was in progress, wait for it to complete */
if (old == Q_REQUEUE_PI_IN_PROGRESS) {
#ifdef CONFIG_PREEMPT_RT
rcuwait_wait_event(&q->requeue_wait,
atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT,
TASK_UNINTERRUPTIBLE);
#else
(void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT);
#endif
}
/*
* Requeue is now either prohibited or complete. Reread state
* because during the wait above it might have changed. Nothing
* will modify q->requeue_state after this point.
*/
return atomic_read(&q->requeue_state);
}
/**
* requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
* @q: the futex_q
* @key: the key of the requeue target futex
* @hb: the hash_bucket of the requeue target futex
*
* During futex_requeue, with requeue_pi=1, it is possible to acquire the
* target futex if it is uncontended or via a lock steal.
*
* 1) Set @q::key to the requeue target futex key so the waiter can detect
* the wakeup on the right futex.
*
* 2) Dequeue @q from the hash bucket.
*
* 3) Set @q::rt_waiter to NULL so the woken up task can detect atomic lock
* acquisition.
*
* 4) Set the q->lock_ptr to the requeue target hb->lock for the case that
* the waiter has to fixup the pi state.
*
* 5) Complete the requeue state so the waiter can make progress. After
* this point the waiter task can return from the syscall immediately in
* case that the pi state does not have to be fixed up.
*
* 6) Wake the waiter task.
*
* Must be called with both q->lock_ptr and hb->lock held.
*/
static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
struct futex_hash_bucket *hb)
{
q->key = *key;
__futex_unqueue(q);
WARN_ON(!q->rt_waiter);
q->rt_waiter = NULL;
q->lock_ptr = &hb->lock;
/* Signal locked state to the waiter */
futex_requeue_pi_complete(q, 1);
wake_up_state(q->task, TASK_NORMAL);
}
/**
* futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
* @pifutex: the user address of the to futex
* @hb1: the from futex hash bucket, must be locked by the caller
* @hb2: the to futex hash bucket, must be locked by the caller
* @key1: the from futex key
* @key2: the to futex key
* @ps: address to store the pi_state pointer
* @exiting: Pointer to store the task pointer of the owner task
* which is in the middle of exiting
* @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
*
* Try and get the lock on behalf of the top waiter if we can do it atomically.
* Wake the top waiter if we succeed. If the caller specified set_waiters,
* then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
* hb1 and hb2 must be held by the caller.
*
* @exiting is only set when the return value is -EBUSY. If so, this holds
* a refcount on the exiting task on return and the caller needs to drop it
* after waiting for the exit to complete.
*
* Return:
* - 0 - failed to acquire the lock atomically;
* - >0 - acquired the lock, return value is vpid of the top_waiter
* - <0 - error
*/
static int
futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2, union futex_key *key1,
union futex_key *key2, struct futex_pi_state **ps,
struct task_struct **exiting, int set_waiters)
{
struct futex_q *top_waiter = NULL;
u32 curval;
int ret;
if (futex_get_value_locked(&curval, pifutex))
return -EFAULT;
if (unlikely(should_fail_futex(true)))
return -EFAULT;
/*
* Find the top_waiter and determine if there are additional waiters.
* If the caller intends to requeue more than 1 waiter to pifutex,
* force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
* as we have means to handle the possible fault. If not, don't set
* the bit unnecessarily as it will force the subsequent unlock to enter
* the kernel.
*/
top_waiter = futex_top_waiter(hb1, key1);
/* There are no waiters, nothing for us to do. */
if (!top_waiter)
return 0;
/*
* Ensure that this is a waiter sitting in futex_wait_requeue_pi()
* and waiting on the 'waitqueue' futex which is always !PI.
*/
if (!top_waiter->rt_waiter || top_waiter->pi_state)
return -EINVAL;
/* Ensure we requeue to the expected futex. */
if (!futex_match(top_waiter->requeue_pi_key, key2))
return -EINVAL;
/* Ensure that this does not race against an early wakeup */
if (!futex_requeue_pi_prepare(top_waiter, NULL))
return -EAGAIN;
/*
* Try to take the lock for top_waiter and set the FUTEX_WAITERS bit
* in the contended case or if @set_waiters is true.
*
* In the contended case PI state is attached to the lock owner. If
* the user space lock can be acquired then PI state is attached to
* the new owner (@top_waiter->task) when @set_waiters is true.
*/
ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
exiting, set_waiters);
if (ret == 1) {
/*
* Lock was acquired in user space and PI state was
* attached to @top_waiter->task. That means state is fully
* consistent and the waiter can return to user space
* immediately after the wakeup.
*/
requeue_pi_wake_futex(top_waiter, key2, hb2);
} else if (ret < 0) {
/* Rewind top_waiter::requeue_state */
futex_requeue_pi_complete(top_waiter, ret);
} else {
/*
* futex_lock_pi_atomic() did not acquire the user space
* futex, but managed to establish the proxy lock and pi
* state. top_waiter::requeue_state cannot be fixed up here
* because the waiter is not enqueued on the rtmutex
* yet. This is handled at the callsite depending on the
* result of rt_mutex_start_proxy_lock() which is
* guaranteed to be reached with this function returning 0.
*/
}
return ret;
}
/**
* futex_requeue() - Requeue waiters from uaddr1 to uaddr2
* @uaddr1: source futex user address
* @flags: futex flags (FLAGS_SHARED, etc.)
* @uaddr2: target futex user address
* @nr_wake: number of waiters to wake (must be 1 for requeue_pi)
* @nr_requeue: number of waiters to requeue (0-INT_MAX)
* @cmpval: @uaddr1 expected value (or %NULL)
* @requeue_pi: if we are attempting to requeue from a non-pi futex to a
* pi futex (pi to pi requeue is not supported)
*
* Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
* uaddr2 atomically on behalf of the top waiter.
*
* Return:
* - >=0 - on success, the number of tasks requeued or woken;
* - <0 - on error
*/
int futex_requeue(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
int nr_wake, int nr_requeue, u32 *cmpval, int requeue_pi)
{
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
int task_count = 0, ret;
struct futex_pi_state *pi_state = NULL;
struct futex_hash_bucket *hb1, *hb2;
struct futex_q *this, *next;
DEFINE_WAKE_Q(wake_q);
if (nr_wake < 0 || nr_requeue < 0)
return -EINVAL;
/*
* When PI not supported: return -ENOSYS if requeue_pi is true,
* consequently the compiler knows requeue_pi is always false past
* this point which will optimize away all the conditional code
* further down.
*/
if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
return -ENOSYS;
if (requeue_pi) {
/*
* Requeue PI only works on two distinct uaddrs. This
* check is only valid for private futexes. See below.
*/
if (uaddr1 == uaddr2)
return -EINVAL;
/*
* futex_requeue() allows the caller to define the number
* of waiters to wake up via the @nr_wake argument. With
* REQUEUE_PI, waking up more than one waiter is creating
* more problems than it solves. Waking up a waiter makes
* only sense if the PI futex @uaddr2 is uncontended as
* this allows the requeue code to acquire the futex
* @uaddr2 before waking the waiter. The waiter can then
* return to user space without further action. A secondary
* wakeup would just make the futex_wait_requeue_pi()
* handling more complex, because that code would have to
* look up pi_state and do more or less all the handling
* which the requeue code has to do for the to be requeued
* waiters. So restrict the number of waiters to wake to
* one, and only wake it up when the PI futex is
* uncontended. Otherwise requeue it and let the unlock of
* the PI futex handle the wakeup.
*
* All REQUEUE_PI users, e.g. pthread_cond_signal() and
* pthread_cond_broadcast() must use nr_wake=1.
*/
if (nr_wake != 1)
return -EINVAL;
/*
* requeue_pi requires a pi_state, try to allocate it now
* without any locks in case it fails.
*/
if (refill_pi_state_cache())
return -ENOMEM;
}
retry:
ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, FUTEX_READ);
if (unlikely(ret != 0))
return ret;
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
requeue_pi ? FUTEX_WRITE : FUTEX_READ);
if (unlikely(ret != 0))
return ret;
/*
* The check above which compares uaddrs is not sufficient for
* shared futexes. We need to compare the keys:
*/
if (requeue_pi && futex_match(&key1, &key2))
return -EINVAL;
hb1 = futex_hash(&key1);
hb2 = futex_hash(&key2);
retry_private:
futex_hb_waiters_inc(hb2);
double_lock_hb(hb1, hb2);
if (likely(cmpval != NULL)) {
u32 curval;
ret = futex_get_value_locked(&curval, uaddr1);
if (unlikely(ret)) {
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
ret = get_user(curval, uaddr1);
if (ret)
return ret;
if (!(flags & FLAGS_SHARED))
goto retry_private;
goto retry;
}
if (curval != *cmpval) {
ret = -EAGAIN;
goto out_unlock;
}
}
if (requeue_pi) {
struct task_struct *exiting = NULL;
/*
* Attempt to acquire uaddr2 and wake the top waiter. If we
* intend to requeue waiters, force setting the FUTEX_WAITERS
* bit. We force this here where we are able to easily handle
* faults rather in the requeue loop below.
*
* Updates topwaiter::requeue_state if a top waiter exists.
*/
ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
&key2, &pi_state,
&exiting, nr_requeue);
/*
* At this point the top_waiter has either taken uaddr2 or
* is waiting on it. In both cases pi_state has been
* established and an initial refcount on it. In case of an
* error there's nothing.
*
* The top waiter's requeue_state is up to date:
*
* - If the lock was acquired atomically (ret == 1), then
* the state is Q_REQUEUE_PI_LOCKED.
*
* The top waiter has been dequeued and woken up and can
* return to user space immediately. The kernel/user
* space state is consistent. In case that there must be
* more waiters requeued the WAITERS bit in the user
* space futex is set so the top waiter task has to go
* into the syscall slowpath to unlock the futex. This
* will block until this requeue operation has been
* completed and the hash bucket locks have been
* dropped.
*
* - If the trylock failed with an error (ret < 0) then
* the state is either Q_REQUEUE_PI_NONE, i.e. "nothing
* happened", or Q_REQUEUE_PI_IGNORE when there was an
* interleaved early wakeup.
*
* - If the trylock did not succeed (ret == 0) then the
* state is either Q_REQUEUE_PI_IN_PROGRESS or
* Q_REQUEUE_PI_WAIT if an early wakeup interleaved.
* This will be cleaned up in the loop below, which
* cannot fail because futex_proxy_trylock_atomic() did
* the same sanity checks for requeue_pi as the loop
* below does.
*/
switch (ret) {
case 0:
/* We hold a reference on the pi state. */
break;
case 1:
/*
* futex_proxy_trylock_atomic() acquired the user space
* futex. Adjust task_count.
*/
task_count++;
ret = 0;
break;
/*
* If the above failed, then pi_state is NULL and
* waiter::requeue_state is correct.
*/
case -EFAULT:
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
ret = fault_in_user_writeable(uaddr2);
if (!ret)
goto retry;
return ret;
case -EBUSY:
case -EAGAIN:
/*
* Two reasons for this:
* - EBUSY: Owner is exiting and we just wait for the
* exit to complete.
* - EAGAIN: The user space value changed.
*/
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
/*
* Handle the case where the owner is in the middle of
* exiting. Wait for the exit to complete otherwise
* this task might loop forever, aka. live lock.
*/
wait_for_owner_exiting(ret, exiting);
cond_resched();
goto retry;
default:
goto out_unlock;
}
}
plist_for_each_entry_safe(this, next, &hb1->chain, list) {
if (task_count - nr_wake >= nr_requeue)
break;
if (!futex_match(&this->key, &key1))
continue;
/*
* FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI should always
* be paired with each other and no other futex ops.
*
* We should never be requeueing a futex_q with a pi_state,
* which is awaiting a futex_unlock_pi().
*/
if ((requeue_pi && !this->rt_waiter) ||
(!requeue_pi && this->rt_waiter) ||
this->pi_state) {
ret = -EINVAL;
break;
}
/* Plain futexes just wake or requeue and are done */
if (!requeue_pi) {
if (++task_count <= nr_wake)
futex_wake_mark(&wake_q, this);
else
requeue_futex(this, hb1, hb2, &key2);
continue;
}
/* Ensure we requeue to the expected futex for requeue_pi. */
if (!futex_match(this->requeue_pi_key, &key2)) {
ret = -EINVAL;
break;
}
/*
* Requeue nr_requeue waiters and possibly one more in the case
* of requeue_pi if we couldn't acquire the lock atomically.
*
* Prepare the waiter to take the rt_mutex. Take a refcount
* on the pi_state and store the pointer in the futex_q
* object of the waiter.
*/
get_pi_state(pi_state);
/* Don't requeue when the waiter is already on the way out. */
if (!futex_requeue_pi_prepare(this, pi_state)) {
/*
* Early woken waiter signaled that it is on the
* way out. Drop the pi_state reference and try the
* next waiter. @this->pi_state is still NULL.
*/
put_pi_state(pi_state);
continue;
}
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
this->task);
if (ret == 1) {
/*
* We got the lock. We do neither drop the refcount
* on pi_state nor clear this->pi_state because the
* waiter needs the pi_state for cleaning up the
* user space value. It will drop the refcount
* after doing so. this::requeue_state is updated
* in the wakeup as well.
*/
requeue_pi_wake_futex(this, &key2, hb2);
task_count++;
} else if (!ret) {
/* Waiter is queued, move it to hb2 */
requeue_futex(this, hb1, hb2, &key2);
futex_requeue_pi_complete(this, 0);
task_count++;
} else {
/*
* rt_mutex_start_proxy_lock() detected a potential
* deadlock when we tried to queue that waiter.
* Drop the pi_state reference which we took above
* and remove the pointer to the state from the
* waiters futex_q object.
*/
this->pi_state = NULL;
put_pi_state(pi_state);
futex_requeue_pi_complete(this, ret);
/*
* We stop queueing more waiters and let user space
* deal with the mess.
*/
break;
}
}
/*
* We took an extra initial reference to the pi_state in
* futex_proxy_trylock_atomic(). We need to drop it here again.
*/
put_pi_state(pi_state);
out_unlock:
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);
futex_hb_waiters_dec(hb2);
return ret ? ret : task_count;
}
/* The key must be already stored in q->key. */
struct futex_hash_bucket *futex_q_lock(struct futex_q *q)
__acquires(&hb->lock)
......@@ -1718,8 +983,8 @@ static long futex_wait_restart(struct restart_block *restart);
* @q: the futex_q to queue up on
* @timeout: the prepared hrtimer_sleeper, or null for no timeout
*/
static void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout)
void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout)
{
/*
* The task state is guaranteed to be set before another task can
......@@ -1766,8 +1031,8 @@ static void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
* - 0 - uaddr contains val and hb has been locked;
* - <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
*/
static int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
struct futex_q *q, struct futex_hash_bucket **hb)
int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
struct futex_q *q, struct futex_hash_bucket **hb)
{
u32 uval;
int ret;
......@@ -1900,225 +1165,6 @@ static long futex_wait_restart(struct restart_block *restart)
}
/**
* handle_early_requeue_pi_wakeup() - Handle early wakeup on the initial futex
* @hb: the hash_bucket futex_q was original enqueued on
* @q: the futex_q woken while waiting to be requeued
* @timeout: the timeout associated with the wait (NULL if none)
*
* Determine the cause for the early wakeup.
*
* Return:
* -EWOULDBLOCK or -ETIMEDOUT or -ERESTARTNOINTR
*/
static inline
int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
struct futex_q *q,
struct hrtimer_sleeper *timeout)
{
int ret;
/*
* With the hb lock held, we avoid races while we process the wakeup.
* We only need to hold hb (and not hb2) to ensure atomicity as the
* wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
* It can't be requeued from uaddr2 to something else since we don't
* support a PI aware source futex for requeue.
*/
WARN_ON_ONCE(&hb->lock != q->lock_ptr);
/*
* We were woken prior to requeue by a timeout or a signal.
* Unqueue the futex_q and determine which it was.
*/
plist_del(&q->list, &hb->chain);
futex_hb_waiters_dec(hb);
/* Handle spurious wakeups gracefully */
ret = -EWOULDBLOCK;
if (timeout && !timeout->task)
ret = -ETIMEDOUT;
else if (signal_pending(current))
ret = -ERESTARTNOINTR;
return ret;
}
/**
* futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
* @uaddr: the futex we initially wait on (non-pi)
* @flags: futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
* the same type, no requeueing from private to shared, etc.
* @val: the expected value of uaddr
* @abs_time: absolute timeout
* @bitset: 32 bit wakeup bitset set by userspace, defaults to all
* @uaddr2: the pi futex we will take prior to returning to user-space
*
* The caller will wait on uaddr and will be requeued by futex_requeue() to
* uaddr2 which must be PI aware and unique from uaddr. Normal wakeup will wake
* on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
* userspace. This ensures the rt_mutex maintains an owner when it has waiters;
* without one, the pi logic would not know which task to boost/deboost, if
* there was a need to.
*
* We call schedule in futex_wait_queue() when we enqueue and return there
* via the following--
* 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
* 2) wakeup on uaddr2 after a requeue
* 3) signal
* 4) timeout
*
* If 3, cleanup and return -ERESTARTNOINTR.
*
* If 2, we may then block on trying to take the rt_mutex and return via:
* 5) successful lock
* 6) signal
* 7) timeout
* 8) other lock acquisition failure
*
* If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
*
* If 4 or 7, we cleanup and return with -ETIMEDOUT.
*
* Return:
* - 0 - On success;
* - <0 - On error
*/
int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
u32 val, ktime_t *abs_time, u32 bitset,
u32 __user *uaddr2)
{
struct hrtimer_sleeper timeout, *to;
struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
union futex_key key2 = FUTEX_KEY_INIT;
struct futex_q q = futex_q_init;
struct rt_mutex_base *pi_mutex;
int res, ret;
if (!IS_ENABLED(CONFIG_FUTEX_PI))
return -ENOSYS;
if (uaddr == uaddr2)
return -EINVAL;
if (!bitset)
return -EINVAL;
to = futex_setup_timer(abs_time, &timeout, flags,
current->timer_slack_ns);
/*
* The waiter is allocated on our stack, manipulated by the requeue
* code while we sleep on uaddr.
*/
rt_mutex_init_waiter(&rt_waiter);
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;
q.bitset = bitset;
q.rt_waiter = &rt_waiter;
q.requeue_pi_key = &key2;
/*
* Prepare to wait on uaddr. On success, it holds hb->lock and q
* is initialized.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/*
* The check above which compares uaddrs is not sufficient for
* shared futexes. We need to compare the keys:
*/
if (futex_match(&q.key, &key2)) {
futex_q_unlock(hb);
ret = -EINVAL;
goto out;
}
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue(hb, &q, to);
switch (futex_requeue_pi_wakeup_sync(&q)) {
case Q_REQUEUE_PI_IGNORE:
/* The waiter is still on uaddr1 */
spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup(hb, &q, to);
spin_unlock(&hb->lock);
break;
case Q_REQUEUE_PI_LOCKED:
/* The requeue acquired the lock */
if (q.pi_state && (q.pi_state->owner != current)) {
spin_lock(q.lock_ptr);
ret = fixup_pi_owner(uaddr2, &q, true);
/*
* Drop the reference to the pi state which the
* requeue_pi() code acquired for us.
*/
put_pi_state(q.pi_state);
spin_unlock(q.lock_ptr);
/*
* Adjust the return value. It's either -EFAULT or
* success (1) but the caller expects 0 for success.
*/
ret = ret < 0 ? ret : 0;
}
break;
case Q_REQUEUE_PI_DONE:
/* Requeue completed. Current is 'pi_blocked_on' the rtmutex */
pi_mutex = &q.pi_state->pi_mutex;
ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);
/* Current is not longer pi_blocked_on */
spin_lock(q.lock_ptr);
if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
ret = 0;
debug_rt_mutex_free_waiter(&rt_waiter);
/*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
*/
res = fixup_pi_owner(uaddr2, &q, !ret);
/*
* If fixup_pi_owner() returned an error, propagate that. If it
* acquired the lock, clear -ETIMEDOUT or -EINTR.
*/
if (res)
ret = (res < 0) ? res : 0;
futex_unqueue_pi(&q);
spin_unlock(q.lock_ptr);
if (ret == -EINTR) {
/*
* We've already been requeued, but cannot restart
* by calling futex_lock_pi() directly. We could
* restart this syscall, but it would detect that
* the user space "val" changed and return
* -EWOULDBLOCK. Save the overhead of the restart
* and return -EWOULDBLOCK directly.
*/
ret = -EWOULDBLOCK;
}
break;
default:
BUG();
}
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret;
}
/* Constants for the pending_op argument of handle_futex_death */
#define HANDLE_DEATH_PENDING true
#define HANDLE_DEATH_LIST false
......
kernel/futex/futex.h
View file @ e5c68284
......@@ -3,6 +3,8 @@
#define _FUTEX_H
#include <linux/futex.h>
#include <linux/sched/wake_q.h>
#include <asm/futex.h>
/*
......@@ -118,22 +120,69 @@ enum futex_access {
extern int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
enum futex_access rw);
extern struct futex_hash_bucket *futex_hash(union futex_key *key);
extern struct hrtimer_sleeper *
futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
int flags, u64 range_ns);
extern struct futex_hash_bucket *futex_hash(union futex_key *key);
/**
* futex_match - Check whether two futex keys are equal
* @key1: Pointer to key1
* @key2: Pointer to key2
*
* Return 1 if two futex_keys are equal, 0 otherwise.
*/
static inline int futex_match(union futex_key *key1, union futex_key *key2)
{
return (key1 && key2
&& key1->both.word == key2->both.word
&& key1->both.ptr == key2->both.ptr
&& key1->both.offset == key2->both.offset);
}
extern int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
struct futex_q *q, struct futex_hash_bucket **hb);
extern void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout);
extern void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q);
extern int fault_in_user_writeable(u32 __user *uaddr);
extern int futex_cmpxchg_value_locked(u32 *curval, u32 __user *uaddr, u32 uval, u32 newval);
extern int futex_get_value_locked(u32 *dest, u32 __user *from);
extern struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb, union futex_key *key);
extern void __futex_unqueue(struct futex_q *q);
extern void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb);
extern void futex_unqueue_pi(struct futex_q *q);
extern void wait_for_owner_exiting(int ret, struct task_struct *exiting);
/*
* Reflects a new waiter being added to the waitqueue.
*/
static inline void futex_hb_waiters_inc(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
atomic_inc(&hb->waiters);
/*
* Full barrier (A), see the ordering comment above.
*/
smp_mb__after_atomic();
#endif
}
/*
* Reflects a waiter being removed from the waitqueue by wakeup
* paths.
*/
static inline void futex_hb_waiters_dec(struct futex_hash_bucket *hb)
{
#ifdef CONFIG_SMP
atomic_dec(&hb->waiters);
#endif
}
extern struct futex_hash_bucket *futex_q_lock(struct futex_q *q);
extern void futex_q_unlock(struct futex_hash_bucket *hb);
......@@ -150,6 +199,30 @@ extern void get_pi_state(struct futex_pi_state *pi_state);
extern void put_pi_state(struct futex_pi_state *pi_state);
extern int fixup_pi_owner(u32 __user *uaddr, struct futex_q *q, int locked);
/*
* Express the locking dependencies for lockdep:
*/
static inline void
double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
if (hb1 <= hb2) {
spin_lock(&hb1->lock);
if (hb1 < hb2)
spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING);
} else { /* hb1 > hb2 */
spin_lock(&hb2->lock);
spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING);
}
}
static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
spin_unlock(&hb1->lock);
if (hb1 != hb2)
spin_unlock(&hb2->lock);
}
/* syscalls */
extern int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, u32
......
kernel/futex/requeue.c 0 → 100644
View file @ e5c68284
// SPDX-License-Identifier: GPL-2.0-or-later
#include <linux/sched/signal.h>
#include "futex.h"
#include "../locking/rtmutex_common.h"
/*
* On PREEMPT_RT, the hash bucket lock is a 'sleeping' spinlock with an
* underlying rtmutex. The task which is about to be requeued could have
* just woken up (timeout, signal). After the wake up the task has to
* acquire hash bucket lock, which is held by the requeue code. As a task
* can only be blocked on _ONE_ rtmutex at a time, the proxy lock blocking
* and the hash bucket lock blocking would collide and corrupt state.
*
* On !PREEMPT_RT this is not a problem and everything could be serialized
* on hash bucket lock, but aside of having the benefit of common code,
* this allows to avoid doing the requeue when the task is already on the
* way out and taking the hash bucket lock of the original uaddr1 when the
* requeue has been completed.
*
* The following state transitions are valid:
*
* On the waiter side:
* Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_IGNORE
* Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_WAIT
*
* On the requeue side:
* Q_REQUEUE_PI_NONE -> Q_REQUEUE_PI_INPROGRESS
* Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_DONE/LOCKED
* Q_REQUEUE_PI_IN_PROGRESS -> Q_REQUEUE_PI_NONE (requeue failed)
* Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_DONE/LOCKED
* Q_REQUEUE_PI_WAIT -> Q_REQUEUE_PI_IGNORE (requeue failed)
*
* The requeue side ignores a waiter with state Q_REQUEUE_PI_IGNORE as this
* signals that the waiter is already on the way out. It also means that
* the waiter is still on the 'wait' futex, i.e. uaddr1.
*
* The waiter side signals early wakeup to the requeue side either through
* setting state to Q_REQUEUE_PI_IGNORE or to Q_REQUEUE_PI_WAIT depending
* on the current state. In case of Q_REQUEUE_PI_IGNORE it can immediately
* proceed to take the hash bucket lock of uaddr1. If it set state to WAIT,
* which means the wakeup is interleaving with a requeue in progress it has
* to wait for the requeue side to change the state. Either to DONE/LOCKED
* or to IGNORE. DONE/LOCKED means the waiter q is now on the uaddr2 futex
* and either blocked (DONE) or has acquired it (LOCKED). IGNORE is set by
* the requeue side when the requeue attempt failed via deadlock detection
* and therefore the waiter q is still on the uaddr1 futex.
*/
enum {
Q_REQUEUE_PI_NONE = 0,
Q_REQUEUE_PI_IGNORE,
Q_REQUEUE_PI_IN_PROGRESS,
Q_REQUEUE_PI_WAIT,
Q_REQUEUE_PI_DONE,
Q_REQUEUE_PI_LOCKED,
};
const struct futex_q futex_q_init = {
/* list gets initialized in futex_queue()*/
.key = FUTEX_KEY_INIT,
.bitset = FUTEX_BITSET_MATCH_ANY,
.requeue_state = ATOMIC_INIT(Q_REQUEUE_PI_NONE),
};
/**
* requeue_futex() - Requeue a futex_q from one hb to another
* @q: the futex_q to requeue
* @hb1: the source hash_bucket
* @hb2: the target hash_bucket
* @key2: the new key for the requeued futex_q
*/
static inline
void requeue_futex(struct futex_q *q, struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2, union futex_key *key2)
{
/*
* If key1 and key2 hash to the same bucket, no need to
* requeue.
*/
if (likely(&hb1->chain != &hb2->chain)) {
plist_del(&q->list, &hb1->chain);
futex_hb_waiters_dec(hb1);
futex_hb_waiters_inc(hb2);
plist_add(&q->list, &hb2->chain);
q->lock_ptr = &hb2->lock;
}
q->key = *key2;
}
static inline bool futex_requeue_pi_prepare(struct futex_q *q,
struct futex_pi_state *pi_state)
{
int old, new;
/*
* Set state to Q_REQUEUE_PI_IN_PROGRESS unless an early wakeup has
* already set Q_REQUEUE_PI_IGNORE to signal that requeue should
* ignore the waiter.
*/
old = atomic_read_acquire(&q->requeue_state);
do {
if (old == Q_REQUEUE_PI_IGNORE)
return false;
/*
* futex_proxy_trylock_atomic() might have set it to
* IN_PROGRESS and a interleaved early wake to WAIT.
*
* It was considered to have an extra state for that
* trylock, but that would just add more conditionals
* all over the place for a dubious value.
*/
if (old != Q_REQUEUE_PI_NONE)
break;
new = Q_REQUEUE_PI_IN_PROGRESS;
} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
q->pi_state = pi_state;
return true;
}
static inline void futex_requeue_pi_complete(struct futex_q *q, int locked)
{
int old, new;
old = atomic_read_acquire(&q->requeue_state);
do {
if (old == Q_REQUEUE_PI_IGNORE)
return;
if (locked >= 0) {
/* Requeue succeeded. Set DONE or LOCKED */
WARN_ON_ONCE(old != Q_REQUEUE_PI_IN_PROGRESS &&
old != Q_REQUEUE_PI_WAIT);
new = Q_REQUEUE_PI_DONE + locked;
} else if (old == Q_REQUEUE_PI_IN_PROGRESS) {
/* Deadlock, no early wakeup interleave */
new = Q_REQUEUE_PI_NONE;
} else {
/* Deadlock, early wakeup interleave. */
WARN_ON_ONCE(old != Q_REQUEUE_PI_WAIT);
new = Q_REQUEUE_PI_IGNORE;
}
} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
#ifdef CONFIG_PREEMPT_RT
/* If the waiter interleaved with the requeue let it know */
if (unlikely(old == Q_REQUEUE_PI_WAIT))
rcuwait_wake_up(&q->requeue_wait);
#endif
}
static inline int futex_requeue_pi_wakeup_sync(struct futex_q *q)
{
int old, new;
old = atomic_read_acquire(&q->requeue_state);
do {
/* Is requeue done already? */
if (old >= Q_REQUEUE_PI_DONE)
return old;
/*
* If not done, then tell the requeue code to either ignore
* the waiter or to wake it up once the requeue is done.
*/
new = Q_REQUEUE_PI_WAIT;
if (old == Q_REQUEUE_PI_NONE)
new = Q_REQUEUE_PI_IGNORE;
} while (!atomic_try_cmpxchg(&q->requeue_state, &old, new));
/* If the requeue was in progress, wait for it to complete */
if (old == Q_REQUEUE_PI_IN_PROGRESS) {
#ifdef CONFIG_PREEMPT_RT
rcuwait_wait_event(&q->requeue_wait,
atomic_read(&q->requeue_state) != Q_REQUEUE_PI_WAIT,
TASK_UNINTERRUPTIBLE);
#else
(void)atomic_cond_read_relaxed(&q->requeue_state, VAL != Q_REQUEUE_PI_WAIT);
#endif
}
/*
* Requeue is now either prohibited or complete. Reread state
* because during the wait above it might have changed. Nothing
* will modify q->requeue_state after this point.
*/
return atomic_read(&q->requeue_state);
}
/**
* requeue_pi_wake_futex() - Wake a task that acquired the lock during requeue
* @q: the futex_q
* @key: the key of the requeue target futex
* @hb: the hash_bucket of the requeue target futex
*
* During futex_requeue, with requeue_pi=1, it is possible to acquire the
* target futex if it is uncontended or via a lock steal.
*
* 1) Set @q::key to the requeue target futex key so the waiter can detect
* the wakeup on the right futex.
*
* 2) Dequeue @q from the hash bucket.
*
* 3) Set @q::rt_waiter to NULL so the woken up task can detect atomic lock
* acquisition.
*
* 4) Set the q->lock_ptr to the requeue target hb->lock for the case that
* the waiter has to fixup the pi state.
*
* 5) Complete the requeue state so the waiter can make progress. After
* this point the waiter task can return from the syscall immediately in
* case that the pi state does not have to be fixed up.
*
* 6) Wake the waiter task.
*
* Must be called with both q->lock_ptr and hb->lock held.
*/
static inline
void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key,
struct futex_hash_bucket *hb)
{
q->key = *key;
__futex_unqueue(q);
WARN_ON(!q->rt_waiter);
q->rt_waiter = NULL;
q->lock_ptr = &hb->lock;
/* Signal locked state to the waiter */
futex_requeue_pi_complete(q, 1);
wake_up_state(q->task, TASK_NORMAL);
}
/**
* futex_proxy_trylock_atomic() - Attempt an atomic lock for the top waiter
* @pifutex: the user address of the to futex
* @hb1: the from futex hash bucket, must be locked by the caller
* @hb2: the to futex hash bucket, must be locked by the caller
* @key1: the from futex key
* @key2: the to futex key
* @ps: address to store the pi_state pointer
* @exiting: Pointer to store the task pointer of the owner task
* which is in the middle of exiting
* @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
*
* Try and get the lock on behalf of the top waiter if we can do it atomically.
* Wake the top waiter if we succeed. If the caller specified set_waiters,
* then direct futex_lock_pi_atomic() to force setting the FUTEX_WAITERS bit.
* hb1 and hb2 must be held by the caller.
*
* @exiting is only set when the return value is -EBUSY. If so, this holds
* a refcount on the exiting task on return and the caller needs to drop it
* after waiting for the exit to complete.
*
* Return:
* - 0 - failed to acquire the lock atomically;
* - >0 - acquired the lock, return value is vpid of the top_waiter
* - <0 - error
*/
static int
futex_proxy_trylock_atomic(u32 __user *pifutex, struct futex_hash_bucket *hb1,
struct futex_hash_bucket *hb2, union futex_key *key1,
union futex_key *key2, struct futex_pi_state **ps,
struct task_struct **exiting, int set_waiters)
{
struct futex_q *top_waiter = NULL;
u32 curval;
int ret;
if (futex_get_value_locked(&curval, pifutex))
return -EFAULT;
if (unlikely(should_fail_futex(true)))
return -EFAULT;
/*
* Find the top_waiter and determine if there are additional waiters.
* If the caller intends to requeue more than 1 waiter to pifutex,
* force futex_lock_pi_atomic() to set the FUTEX_WAITERS bit now,
* as we have means to handle the possible fault. If not, don't set
* the bit unnecessarily as it will force the subsequent unlock to enter
* the kernel.
*/
top_waiter = futex_top_waiter(hb1, key1);
/* There are no waiters, nothing for us to do. */
if (!top_waiter)
return 0;
/*
* Ensure that this is a waiter sitting in futex_wait_requeue_pi()
* and waiting on the 'waitqueue' futex which is always !PI.
*/
if (!top_waiter->rt_waiter || top_waiter->pi_state)
return -EINVAL;
/* Ensure we requeue to the expected futex. */
if (!futex_match(top_waiter->requeue_pi_key, key2))
return -EINVAL;
/* Ensure that this does not race against an early wakeup */
if (!futex_requeue_pi_prepare(top_waiter, NULL))
return -EAGAIN;
/*
* Try to take the lock for top_waiter and set the FUTEX_WAITERS bit
* in the contended case or if @set_waiters is true.
*
* In the contended case PI state is attached to the lock owner. If
* the user space lock can be acquired then PI state is attached to
* the new owner (@top_waiter->task) when @set_waiters is true.
*/
ret = futex_lock_pi_atomic(pifutex, hb2, key2, ps, top_waiter->task,
exiting, set_waiters);
if (ret == 1) {
/*
* Lock was acquired in user space and PI state was
* attached to @top_waiter->task. That means state is fully
* consistent and the waiter can return to user space
* immediately after the wakeup.
*/
requeue_pi_wake_futex(top_waiter, key2, hb2);
} else if (ret < 0) {
/* Rewind top_waiter::requeue_state */
futex_requeue_pi_complete(top_waiter, ret);
} else {
/*
* futex_lock_pi_atomic() did not acquire the user space
* futex, but managed to establish the proxy lock and pi
* state. top_waiter::requeue_state cannot be fixed up here
* because the waiter is not enqueued on the rtmutex
* yet. This is handled at the callsite depending on the
* result of rt_mutex_start_proxy_lock() which is
* guaranteed to be reached with this function returning 0.
*/
}
return ret;
}
/**
* futex_requeue() - Requeue waiters from uaddr1 to uaddr2
* @uaddr1: source futex user address
* @flags: futex flags (FLAGS_SHARED, etc.)
* @uaddr2: target futex user address
* @nr_wake: number of waiters to wake (must be 1 for requeue_pi)
* @nr_requeue: number of waiters to requeue (0-INT_MAX)
* @cmpval: @uaddr1 expected value (or %NULL)
* @requeue_pi: if we are attempting to requeue from a non-pi futex to a
* pi futex (pi to pi requeue is not supported)
*
* Requeue waiters on uaddr1 to uaddr2. In the requeue_pi case, try to acquire
* uaddr2 atomically on behalf of the top waiter.
*
* Return:
* - >=0 - on success, the number of tasks requeued or woken;
* - <0 - on error
*/
int futex_requeue(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
int nr_wake, int nr_requeue, u32 *cmpval, int requeue_pi)
{
union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
int task_count = 0, ret;
struct futex_pi_state *pi_state = NULL;
struct futex_hash_bucket *hb1, *hb2;
struct futex_q *this, *next;
DEFINE_WAKE_Q(wake_q);
if (nr_wake < 0 || nr_requeue < 0)
return -EINVAL;
/*
* When PI not supported: return -ENOSYS if requeue_pi is true,
* consequently the compiler knows requeue_pi is always false past
* this point which will optimize away all the conditional code
* further down.
*/
if (!IS_ENABLED(CONFIG_FUTEX_PI) && requeue_pi)
return -ENOSYS;
if (requeue_pi) {
/*
* Requeue PI only works on two distinct uaddrs. This
* check is only valid for private futexes. See below.
*/
if (uaddr1 == uaddr2)
return -EINVAL;
/*
* futex_requeue() allows the caller to define the number
* of waiters to wake up via the @nr_wake argument. With
* REQUEUE_PI, waking up more than one waiter is creating
* more problems than it solves. Waking up a waiter makes
* only sense if the PI futex @uaddr2 is uncontended as
* this allows the requeue code to acquire the futex
* @uaddr2 before waking the waiter. The waiter can then
* return to user space without further action. A secondary
* wakeup would just make the futex_wait_requeue_pi()
* handling more complex, because that code would have to
* look up pi_state and do more or less all the handling
* which the requeue code has to do for the to be requeued
* waiters. So restrict the number of waiters to wake to
* one, and only wake it up when the PI futex is
* uncontended. Otherwise requeue it and let the unlock of
* the PI futex handle the wakeup.
*
* All REQUEUE_PI users, e.g. pthread_cond_signal() and
* pthread_cond_broadcast() must use nr_wake=1.
*/
if (nr_wake != 1)
return -EINVAL;
/*
* requeue_pi requires a pi_state, try to allocate it now
* without any locks in case it fails.
*/
if (refill_pi_state_cache())
return -ENOMEM;
}
retry:
ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, FUTEX_READ);
if (unlikely(ret != 0))
return ret;
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2,
requeue_pi ? FUTEX_WRITE : FUTEX_READ);
if (unlikely(ret != 0))
return ret;
/*
* The check above which compares uaddrs is not sufficient for
* shared futexes. We need to compare the keys:
*/
if (requeue_pi && futex_match(&key1, &key2))
return -EINVAL;
hb1 = futex_hash(&key1);
hb2 = futex_hash(&key2);
retry_private:
futex_hb_waiters_inc(hb2);
double_lock_hb(hb1, hb2);
if (likely(cmpval != NULL)) {
u32 curval;
ret = futex_get_value_locked(&curval, uaddr1);
if (unlikely(ret)) {
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
ret = get_user(curval, uaddr1);
if (ret)
return ret;
if (!(flags & FLAGS_SHARED))
goto retry_private;
goto retry;
}
if (curval != *cmpval) {
ret = -EAGAIN;
goto out_unlock;
}
}
if (requeue_pi) {
struct task_struct *exiting = NULL;
/*
* Attempt to acquire uaddr2 and wake the top waiter. If we
* intend to requeue waiters, force setting the FUTEX_WAITERS
* bit. We force this here where we are able to easily handle
* faults rather in the requeue loop below.
*
* Updates topwaiter::requeue_state if a top waiter exists.
*/
ret = futex_proxy_trylock_atomic(uaddr2, hb1, hb2, &key1,
&key2, &pi_state,
&exiting, nr_requeue);
/*
* At this point the top_waiter has either taken uaddr2 or
* is waiting on it. In both cases pi_state has been
* established and an initial refcount on it. In case of an
* error there's nothing.
*
* The top waiter's requeue_state is up to date:
*
* - If the lock was acquired atomically (ret == 1), then
* the state is Q_REQUEUE_PI_LOCKED.
*
* The top waiter has been dequeued and woken up and can
* return to user space immediately. The kernel/user
* space state is consistent. In case that there must be
* more waiters requeued the WAITERS bit in the user
* space futex is set so the top waiter task has to go
* into the syscall slowpath to unlock the futex. This
* will block until this requeue operation has been
* completed and the hash bucket locks have been
* dropped.
*
* - If the trylock failed with an error (ret < 0) then
* the state is either Q_REQUEUE_PI_NONE, i.e. "nothing
* happened", or Q_REQUEUE_PI_IGNORE when there was an
* interleaved early wakeup.
*
* - If the trylock did not succeed (ret == 0) then the
* state is either Q_REQUEUE_PI_IN_PROGRESS or
* Q_REQUEUE_PI_WAIT if an early wakeup interleaved.
* This will be cleaned up in the loop below, which
* cannot fail because futex_proxy_trylock_atomic() did
* the same sanity checks for requeue_pi as the loop
* below does.
*/
switch (ret) {
case 0:
/* We hold a reference on the pi state. */
break;
case 1:
/*
* futex_proxy_trylock_atomic() acquired the user space
* futex. Adjust task_count.
*/
task_count++;
ret = 0;
break;
/*
* If the above failed, then pi_state is NULL and
* waiter::requeue_state is correct.
*/
case -EFAULT:
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
ret = fault_in_user_writeable(uaddr2);
if (!ret)
goto retry;
return ret;
case -EBUSY:
case -EAGAIN:
/*
* Two reasons for this:
* - EBUSY: Owner is exiting and we just wait for the
* exit to complete.
* - EAGAIN: The user space value changed.
*/
double_unlock_hb(hb1, hb2);
futex_hb_waiters_dec(hb2);
/*
* Handle the case where the owner is in the middle of
* exiting. Wait for the exit to complete otherwise
* this task might loop forever, aka. live lock.
*/
wait_for_owner_exiting(ret, exiting);
cond_resched();
goto retry;
default:
goto out_unlock;
}
}
plist_for_each_entry_safe(this, next, &hb1->chain, list) {
if (task_count - nr_wake >= nr_requeue)
break;
if (!futex_match(&this->key, &key1))
continue;
/*
* FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI should always
* be paired with each other and no other futex ops.
*
* We should never be requeueing a futex_q with a pi_state,
* which is awaiting a futex_unlock_pi().
*/
if ((requeue_pi && !this->rt_waiter) ||
(!requeue_pi && this->rt_waiter) ||
this->pi_state) {
ret = -EINVAL;
break;
}
/* Plain futexes just wake or requeue and are done */
if (!requeue_pi) {
if (++task_count <= nr_wake)
futex_wake_mark(&wake_q, this);
else
requeue_futex(this, hb1, hb2, &key2);
continue;
}
/* Ensure we requeue to the expected futex for requeue_pi. */
if (!futex_match(this->requeue_pi_key, &key2)) {
ret = -EINVAL;
break;
}
/*
* Requeue nr_requeue waiters and possibly one more in the case
* of requeue_pi if we couldn't acquire the lock atomically.
*
* Prepare the waiter to take the rt_mutex. Take a refcount
* on the pi_state and store the pointer in the futex_q
* object of the waiter.
*/
get_pi_state(pi_state);
/* Don't requeue when the waiter is already on the way out. */
if (!futex_requeue_pi_prepare(this, pi_state)) {
/*
* Early woken waiter signaled that it is on the
* way out. Drop the pi_state reference and try the
* next waiter. @this->pi_state is still NULL.
*/
put_pi_state(pi_state);
continue;
}
ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
this->rt_waiter,
this->task);
if (ret == 1) {
/*
* We got the lock. We do neither drop the refcount
* on pi_state nor clear this->pi_state because the
* waiter needs the pi_state for cleaning up the
* user space value. It will drop the refcount
* after doing so. this::requeue_state is updated
* in the wakeup as well.
*/
requeue_pi_wake_futex(this, &key2, hb2);
task_count++;
} else if (!ret) {
/* Waiter is queued, move it to hb2 */
requeue_futex(this, hb1, hb2, &key2);
futex_requeue_pi_complete(this, 0);
task_count++;
} else {
/*
* rt_mutex_start_proxy_lock() detected a potential
* deadlock when we tried to queue that waiter.
* Drop the pi_state reference which we took above
* and remove the pointer to the state from the
* waiters futex_q object.
*/
this->pi_state = NULL;
put_pi_state(pi_state);
futex_requeue_pi_complete(this, ret);
/*
* We stop queueing more waiters and let user space
* deal with the mess.
*/
break;
}
}
/*
* We took an extra initial reference to the pi_state in
* futex_proxy_trylock_atomic(). We need to drop it here again.
*/
put_pi_state(pi_state);
out_unlock:
double_unlock_hb(hb1, hb2);
wake_up_q(&wake_q);
futex_hb_waiters_dec(hb2);
return ret ? ret : task_count;
}
/**
* handle_early_requeue_pi_wakeup() - Handle early wakeup on the initial futex
* @hb: the hash_bucket futex_q was original enqueued on
* @q: the futex_q woken while waiting to be requeued
* @timeout: the timeout associated with the wait (NULL if none)
*
* Determine the cause for the early wakeup.
*
* Return:
* -EWOULDBLOCK or -ETIMEDOUT or -ERESTARTNOINTR
*/
static inline
int handle_early_requeue_pi_wakeup(struct futex_hash_bucket *hb,
struct futex_q *q,
struct hrtimer_sleeper *timeout)
{
int ret;
/*
* With the hb lock held, we avoid races while we process the wakeup.
* We only need to hold hb (and not hb2) to ensure atomicity as the
* wakeup code can't change q.key from uaddr to uaddr2 if we hold hb.
* It can't be requeued from uaddr2 to something else since we don't
* support a PI aware source futex for requeue.
*/
WARN_ON_ONCE(&hb->lock != q->lock_ptr);
/*
* We were woken prior to requeue by a timeout or a signal.
* Unqueue the futex_q and determine which it was.
*/
plist_del(&q->list, &hb->chain);
futex_hb_waiters_dec(hb);
/* Handle spurious wakeups gracefully */
ret = -EWOULDBLOCK;
if (timeout && !timeout->task)
ret = -ETIMEDOUT;
else if (signal_pending(current))
ret = -ERESTARTNOINTR;
return ret;
}
/**
* futex_wait_requeue_pi() - Wait on uaddr and take uaddr2
* @uaddr: the futex we initially wait on (non-pi)
* @flags: futex flags (FLAGS_SHARED, FLAGS_CLOCKRT, etc.), they must be
* the same type, no requeueing from private to shared, etc.
* @val: the expected value of uaddr
* @abs_time: absolute timeout
* @bitset: 32 bit wakeup bitset set by userspace, defaults to all
* @uaddr2: the pi futex we will take prior to returning to user-space
*
* The caller will wait on uaddr and will be requeued by futex_requeue() to
* uaddr2 which must be PI aware and unique from uaddr. Normal wakeup will wake
* on uaddr2 and complete the acquisition of the rt_mutex prior to returning to
* userspace. This ensures the rt_mutex maintains an owner when it has waiters;
* without one, the pi logic would not know which task to boost/deboost, if
* there was a need to.
*
* We call schedule in futex_wait_queue() when we enqueue and return there
* via the following--
* 1) wakeup on uaddr2 after an atomic lock acquisition by futex_requeue()
* 2) wakeup on uaddr2 after a requeue
* 3) signal
* 4) timeout
*
* If 3, cleanup and return -ERESTARTNOINTR.
*
* If 2, we may then block on trying to take the rt_mutex and return via:
* 5) successful lock
* 6) signal
* 7) timeout
* 8) other lock acquisition failure
*
* If 6, return -EWOULDBLOCK (restarting the syscall would do the same).
*
* If 4 or 7, we cleanup and return with -ETIMEDOUT.
*
* Return:
* - 0 - On success;
* - <0 - On error
*/
int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
u32 val, ktime_t *abs_time, u32 bitset,
u32 __user *uaddr2)
{
struct hrtimer_sleeper timeout, *to;
struct rt_mutex_waiter rt_waiter;
struct futex_hash_bucket *hb;
union futex_key key2 = FUTEX_KEY_INIT;
struct futex_q q = futex_q_init;
struct rt_mutex_base *pi_mutex;
int res, ret;
if (!IS_ENABLED(CONFIG_FUTEX_PI))
return -ENOSYS;
if (uaddr == uaddr2)
return -EINVAL;
if (!bitset)
return -EINVAL;
to = futex_setup_timer(abs_time, &timeout, flags,
current->timer_slack_ns);
/*
* The waiter is allocated on our stack, manipulated by the requeue
* code while we sleep on uaddr.
*/
rt_mutex_init_waiter(&rt_waiter);
ret = get_futex_key(uaddr2, flags & FLAGS_SHARED, &key2, FUTEX_WRITE);
if (unlikely(ret != 0))
goto out;
q.bitset = bitset;
q.rt_waiter = &rt_waiter;
q.requeue_pi_key = &key2;
/*
* Prepare to wait on uaddr. On success, it holds hb->lock and q
* is initialized.
*/
ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
if (ret)
goto out;
/*
* The check above which compares uaddrs is not sufficient for
* shared futexes. We need to compare the keys:
*/
if (futex_match(&q.key, &key2)) {
futex_q_unlock(hb);
ret = -EINVAL;
goto out;
}
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue(hb, &q, to);
switch (futex_requeue_pi_wakeup_sync(&q)) {
case Q_REQUEUE_PI_IGNORE:
/* The waiter is still on uaddr1 */
spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup(hb, &q, to);
spin_unlock(&hb->lock);
break;
case Q_REQUEUE_PI_LOCKED:
/* The requeue acquired the lock */
if (q.pi_state && (q.pi_state->owner != current)) {
spin_lock(q.lock_ptr);
ret = fixup_pi_owner(uaddr2, &q, true);
/*
* Drop the reference to the pi state which the
* requeue_pi() code acquired for us.
*/
put_pi_state(q.pi_state);
spin_unlock(q.lock_ptr);
/*
* Adjust the return value. It's either -EFAULT or
* success (1) but the caller expects 0 for success.
*/
ret = ret < 0 ? ret : 0;
}
break;
case Q_REQUEUE_PI_DONE:
/* Requeue completed. Current is 'pi_blocked_on' the rtmutex */
pi_mutex = &q.pi_state->pi_mutex;
ret = rt_mutex_wait_proxy_lock(pi_mutex, to, &rt_waiter);
/* Current is not longer pi_blocked_on */
spin_lock(q.lock_ptr);
if (ret && !rt_mutex_cleanup_proxy_lock(pi_mutex, &rt_waiter))
ret = 0;
debug_rt_mutex_free_waiter(&rt_waiter);
/*
* Fixup the pi_state owner and possibly acquire the lock if we
* haven't already.
*/
res = fixup_pi_owner(uaddr2, &q, !ret);
/*
* If fixup_pi_owner() returned an error, propagate that. If it
* acquired the lock, clear -ETIMEDOUT or -EINTR.
*/
if (res)
ret = (res < 0) ? res : 0;
futex_unqueue_pi(&q);
spin_unlock(q.lock_ptr);
if (ret == -EINTR) {
/*
* We've already been requeued, but cannot restart
* by calling futex_lock_pi() directly. We could
* restart this syscall, but it would detect that
* the user space "val" changed and return
* -EWOULDBLOCK. Save the overhead of the restart
* and return -EWOULDBLOCK directly.
*/
ret = -EWOULDBLOCK;
}
break;
default:
BUG();
}
out:
if (to) {
hrtimer_cancel(&to->timer);
destroy_hrtimer_on_stack(&to->timer);
}
return ret;
}
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