Commit 7f26482a authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

locking/percpu-rwsem: Remove the embedded rwsem

The filesystem freezer uses percpu-rwsem in a way that is effectively
write_non_owner() and achieves this with a few horrible hacks that
rely on the rwsem (!percpu) implementation.

When PREEMPT_RT replaces the rwsem implementation with a PI aware
variant this comes apart.

Remove the embedded rwsem and implement it using a waitqueue and an
atomic_t.

 - make readers_block an atomic, and use it, with the waitqueue
   for a blocking test-and-set write-side.

 - have the read-side wait for the 'lock' state to clear.

Have the waiters use FIFO queueing and mark them (reader/writer) with
a new WQ_FLAG. Use a custom wake_function to wake either a single
writer or all readers until a writer.
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Acked-by: default avatarWill Deacon <will@kernel.org>
Acked-by: default avatarWaiman Long <longman@redhat.com>
Tested-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200204092403.GB14879@hirez.programming.kicks-ass.net
parent 75ff6457
......@@ -3,18 +3,18 @@
#define _LINUX_PERCPU_RWSEM_H
#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/rcuwait.h>
#include <linux/wait.h>
#include <linux/rcu_sync.h>
#include <linux/lockdep.h>
struct percpu_rw_semaphore {
struct rcu_sync rss;
unsigned int __percpu *read_count;
struct rw_semaphore rw_sem; /* slowpath */
struct rcuwait writer; /* blocked writer */
int readers_block;
struct rcuwait writer;
wait_queue_head_t waiters;
atomic_t block;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
......@@ -31,8 +31,9 @@ static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_rc_##name); \
is_static struct percpu_rw_semaphore name = { \
.rss = __RCU_SYNC_INITIALIZER(name.rss), \
.read_count = &__percpu_rwsem_rc_##name, \
.rw_sem = __RWSEM_INITIALIZER(name.rw_sem), \
.writer = __RCUWAIT_INITIALIZER(name.writer), \
.waiters = __WAIT_QUEUE_HEAD_INITIALIZER(name.waiters), \
.block = ATOMIC_INIT(0), \
__PERCPU_RWSEM_DEP_MAP_INIT(name) \
}
......@@ -130,20 +131,12 @@ static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
bool read, unsigned long ip)
{
lock_release(&sem->dep_map, ip);
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
if (!read)
atomic_long_set(&sem->rw_sem.owner, RWSEM_OWNER_UNKNOWN);
#endif
}
static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
bool read, unsigned long ip)
{
lock_acquire(&sem->dep_map, 0, 1, read, 1, NULL, ip);
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
if (!read)
atomic_long_set(&sem->rw_sem.owner, (long)current);
#endif
}
#endif
......@@ -20,6 +20,7 @@ int default_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int
#define WQ_FLAG_EXCLUSIVE 0x01
#define WQ_FLAG_WOKEN 0x02
#define WQ_FLAG_BOOKMARK 0x04
#define WQ_FLAG_CUSTOM 0x08
/*
* A single wait-queue entry structure:
......
// SPDX-License-Identifier: GPL-2.0-only
#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/wait.h>
#include <linux/lockdep.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/errno.h>
#include "rwsem.h"
int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
const char *name, struct lock_class_key *key)
{
......@@ -17,11 +16,10 @@ int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
if (unlikely(!sem->read_count))
return -ENOMEM;
/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
rcu_sync_init(&sem->rss);
init_rwsem(&sem->rw_sem);
rcuwait_init(&sem->writer);
sem->readers_block = 0;
init_waitqueue_head(&sem->waiters);
atomic_set(&sem->block, 0);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
debug_check_no_locks_freed((void *)sem, sizeof(*sem));
lockdep_init_map(&sem->dep_map, name, key, 0);
......@@ -54,23 +52,23 @@ static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
* the same CPU as the increment, avoiding the
* increment-on-one-CPU-and-decrement-on-another problem.
*
* If the reader misses the writer's assignment of readers_block, then
* the writer is guaranteed to see the reader's increment.
* If the reader misses the writer's assignment of sem->block, then the
* writer is guaranteed to see the reader's increment.
*
* Conversely, any readers that increment their sem->read_count after
* the writer looks are guaranteed to see the readers_block value,
* which in turn means that they are guaranteed to immediately
* decrement their sem->read_count, so that it doesn't matter that the
* writer missed them.
* the writer looks are guaranteed to see the sem->block value, which
* in turn means that they are guaranteed to immediately decrement
* their sem->read_count, so that it doesn't matter that the writer
* missed them.
*/
smp_mb(); /* A matches D */
/*
* If !readers_block the critical section starts here, matched by the
* If !sem->block the critical section starts here, matched by the
* release in percpu_up_write().
*/
if (likely(!smp_load_acquire(&sem->readers_block)))
if (likely(!atomic_read_acquire(&sem->block)))
return true;
__this_cpu_dec(*sem->read_count);
......@@ -81,6 +79,88 @@ static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
return false;
}
static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
{
if (atomic_read(&sem->block))
return false;
return atomic_xchg(&sem->block, 1) == 0;
}
static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
{
if (reader) {
bool ret;
preempt_disable();
ret = __percpu_down_read_trylock(sem);
preempt_enable();
return ret;
}
return __percpu_down_write_trylock(sem);
}
/*
* The return value of wait_queue_entry::func means:
*
* <0 - error, wakeup is terminated and the error is returned
* 0 - no wakeup, a next waiter is tried
* >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
*
* We use EXCLUSIVE for both readers and writers to preserve FIFO order,
* and play games with the return value to allow waking multiple readers.
*
* Specifically, we wake readers until we've woken a single writer, or until a
* trylock fails.
*/
static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
unsigned int mode, int wake_flags,
void *key)
{
struct task_struct *p = get_task_struct(wq_entry->private);
bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
struct percpu_rw_semaphore *sem = key;
/* concurrent against percpu_down_write(), can get stolen */
if (!__percpu_rwsem_trylock(sem, reader))
return 1;
list_del_init(&wq_entry->entry);
smp_store_release(&wq_entry->private, NULL);
wake_up_process(p);
put_task_struct(p);
return !reader; /* wake (readers until) 1 writer */
}
static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
{
DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
bool wait;
spin_lock_irq(&sem->waiters.lock);
/*
* Serialize against the wakeup in percpu_up_write(), if we fail
* the trylock, the wakeup must see us on the list.
*/
wait = !__percpu_rwsem_trylock(sem, reader);
if (wait) {
wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
__add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
}
spin_unlock_irq(&sem->waiters.lock);
while (wait) {
set_current_state(TASK_UNINTERRUPTIBLE);
if (!smp_load_acquire(&wq_entry.private))
break;
schedule();
}
__set_current_state(TASK_RUNNING);
}
bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
{
if (__percpu_down_read_trylock(sem))
......@@ -89,20 +169,10 @@ bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
if (try)
return false;
/*
* We either call schedule() in the wait, or we'll fall through
* and reschedule on the preempt_enable() in percpu_down_read().
*/
preempt_enable_no_resched();
/*
* Avoid lockdep for the down/up_read() we already have them.
*/
__down_read(&sem->rw_sem);
this_cpu_inc(*sem->read_count);
__up_read(&sem->rw_sem);
preempt_enable();
percpu_rwsem_wait(sem, /* .reader = */ true);
preempt_disable();
return true;
}
EXPORT_SYMBOL_GPL(__percpu_down_read);
......@@ -117,7 +187,7 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
*/
__this_cpu_dec(*sem->read_count);
/* Prod writer to recheck readers_active */
/* Prod writer to re-evaluate readers_active_check() */
rcuwait_wake_up(&sem->writer);
}
EXPORT_SYMBOL_GPL(__percpu_up_read);
......@@ -137,6 +207,8 @@ EXPORT_SYMBOL_GPL(__percpu_up_read);
* zero. If this sum is zero, then it is stable due to the fact that if any
* newly arriving readers increment a given counter, they will immediately
* decrement that same counter.
*
* Assumes sem->block is set.
*/
static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
......@@ -160,23 +232,22 @@ void percpu_down_write(struct percpu_rw_semaphore *sem)
/* Notify readers to take the slow path. */
rcu_sync_enter(&sem->rss);
__down_write(&sem->rw_sem);
/*
* Notify new readers to block; up until now, and thus throughout the
* longish rcu_sync_enter() above, new readers could still come in.
* Try set sem->block; this provides writer-writer exclusion.
* Having sem->block set makes new readers block.
*/
WRITE_ONCE(sem->readers_block, 1);
if (!__percpu_down_write_trylock(sem))
percpu_rwsem_wait(sem, /* .reader = */ false);
smp_mb(); /* D matches A */
/* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
/*
* If they don't see our writer of readers_block, then we are
* guaranteed to see their sem->read_count increment, and therefore
* will wait for them.
* If they don't see our store of sem->block, then we are guaranteed to
* see their sem->read_count increment, and therefore will wait for
* them.
*/
/* Wait for all now active readers to complete. */
/* Wait for all active readers to complete. */
rcuwait_wait_event(&sem->writer, readers_active_check(sem));
}
EXPORT_SYMBOL_GPL(percpu_down_write);
......@@ -195,12 +266,12 @@ void percpu_up_write(struct percpu_rw_semaphore *sem)
* Therefore we force it through the slow path which guarantees an
* acquire and thereby guarantees the critical section's consistency.
*/
smp_store_release(&sem->readers_block, 0);
atomic_set_release(&sem->block, 0);
/*
* Release the write lock, this will allow readers back in the game.
* Prod any pending reader/writer to make progress.
*/
__up_write(&sem->rw_sem);
__wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
/*
* Once this completes (at least one RCU-sched grace period hence) the
......
......@@ -28,7 +28,6 @@
#include <linux/rwsem.h>
#include <linux/atomic.h>
#include "rwsem.h"
#include "lock_events.h"
/*
......@@ -1338,7 +1337,7 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
/*
* lock for reading
*/
inline void __down_read(struct rw_semaphore *sem)
static inline void __down_read(struct rw_semaphore *sem)
{
if (!rwsem_read_trylock(sem)) {
rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
......@@ -1383,7 +1382,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
/*
* lock for writing
*/
inline void __down_write(struct rw_semaphore *sem)
static inline void __down_write(struct rw_semaphore *sem)
{
long tmp = RWSEM_UNLOCKED_VALUE;
......@@ -1426,7 +1425,7 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
/*
* unlock after reading
*/
inline void __up_read(struct rw_semaphore *sem)
static inline void __up_read(struct rw_semaphore *sem)
{
long tmp;
......@@ -1446,7 +1445,7 @@ inline void __up_read(struct rw_semaphore *sem)
/*
* unlock after writing
*/
inline void __up_write(struct rw_semaphore *sem)
static inline void __up_write(struct rw_semaphore *sem)
{
long tmp;
......
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __INTERNAL_RWSEM_H
#define __INTERNAL_RWSEM_H
#include <linux/rwsem.h>
extern void __down_read(struct rw_semaphore *sem);
extern void __up_read(struct rw_semaphore *sem);
extern void __down_write(struct rw_semaphore *sem);
extern void __up_write(struct rw_semaphore *sem);
#endif /* __INTERNAL_RWSEM_H */
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