Commit 73d786bd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner

futex: Rework inconsistent rt_mutex/futex_q state

There is a weird state in the futex_unlock_pi() path when it interleaves
with a concurrent futex_lock_pi() at the point where it drops hb->lock.

In this case, it can happen that the rt_mutex wait_list and the futex_q
disagree on pending waiters, in particular rt_mutex will find no pending
waiters where futex_q thinks there are. In this case the rt_mutex unlock
code cannot assign an owner.

The futex side fixup code has to cleanup the inconsistencies with quite a
bunch of interesting corner cases.

Simplify all this by changing wake_futex_pi() to return -EAGAIN when this
situation occurs. This then gives the futex_lock_pi() code the opportunity
to continue and the retried futex_unlock_pi() will now observe a coherent
state.

The only problem is that this breaks RT timeliness guarantees. That
is, consider the following scenario:

  T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)

    CPU0

    T1
      lock_pi()
      queue_me()  <- Waiter is visible

    preemption

    T2
      unlock_pi()
	loops with -EAGAIN forever

Which is undesirable for PI primitives. Future patches will rectify
this.
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Link: http://lkml.kernel.org/r/20170322104151.850383690@infradead.orgSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent bf92cf3a
...@@ -1404,12 +1404,19 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter ...@@ -1404,12 +1404,19 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter
new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
/* /*
* It is possible that the next waiter (the one that brought * When we interleave with futex_lock_pi() where it does
* top_waiter owner to the kernel) timed out and is no longer * rt_mutex_timed_futex_lock(), we might observe @this futex_q waiter,
* waiting on the lock. * but the rt_mutex's wait_list can be empty (either still, or again,
* depending on which side we land).
*
* When this happens, give up our locks and try again, giving the
* futex_lock_pi() instance time to complete, either by waiting on the
* rtmutex or removing itself from the futex queue.
*/ */
if (!new_owner) if (!new_owner) {
new_owner = top_waiter->task; raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
return -EAGAIN;
}
/* /*
* We pass it to the next owner. The WAITERS bit is always * We pass it to the next owner. The WAITERS bit is always
...@@ -2332,7 +2339,6 @@ static long futex_wait_restart(struct restart_block *restart); ...@@ -2332,7 +2339,6 @@ static long futex_wait_restart(struct restart_block *restart);
*/ */
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked) static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{ {
struct task_struct *owner;
int ret = 0; int ret = 0;
if (locked) { if (locked) {
...@@ -2345,44 +2351,16 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked) ...@@ -2345,44 +2351,16 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
goto out; goto out;
} }
/*
* Catch the rare case, where the lock was released when we were on the
* way back before we locked the hash bucket.
*/
if (q->pi_state->owner == current) {
/*
* Try to get the rt_mutex now. This might fail as some other
* task acquired the rt_mutex after we removed ourself from the
* rt_mutex waiters list.
*/
if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) {
locked = 1;
goto out;
}
/*
* pi_state is incorrect, some other task did a lock steal and
* we returned due to timeout or signal without taking the
* rt_mutex. Too late.
*/
raw_spin_lock_irq(&q->pi_state->pi_mutex.wait_lock);
owner = rt_mutex_owner(&q->pi_state->pi_mutex);
if (!owner)
owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
raw_spin_unlock_irq(&q->pi_state->pi_mutex.wait_lock);
ret = fixup_pi_state_owner(uaddr, q, owner);
goto out;
}
/* /*
* Paranoia check. If we did not take the lock, then we should not be * Paranoia check. If we did not take the lock, then we should not be
* the owner of the rt_mutex. * the owner of the rt_mutex.
*/ */
if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) {
printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p " printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
"pi-state %p\n", ret, "pi-state %p\n", ret,
q->pi_state->pi_mutex.owner, q->pi_state->pi_mutex.owner,
q->pi_state->owner); q->pi_state->owner);
}
out: out:
return ret ? ret : locked; return ret ? ret : locked;
......
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