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locking/percpu-rwsem: Use this_cpu_{inc,dec}() for read_count 

The __this_cpu*() accessors are (in general) IRQ-unsafe which, given
that percpu-rwsem is a blocking primitive, should be just fine.

However, file_end_write() is used from IRQ context and will cause
load-store issues on architectures where the per-cpu accessors are not
natively irq-safe.

Fix it by using the IRQ-safe this_cpu_*() for operations on
read_count. This will generate more expensive code on a number of
platforms, which might cause a performance regression for some of the
other percpu-rwsem users.

If any such is reported, we can consider alternative solutions.

Fixes: 70fe2f4815 ("aio: fix freeze protection of aio writes")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20200915140750.137881-1-houtao1@huawei.com
This commit is contained in:
Hou Tao 2020-09-15 22:07:50 +08:00 committed by Peter Zijlstra
parent 23870f1227
commit e6b1a44ecc
2 changed files with 6 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
include/linux/percpu-rwsem.h
View File

@ -60,7 +60,7 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
* anything we did within this RCU-sched read-size critical section.
*/
if (likely(rcu_sync_is_idle(&sem->rss)))
__this_cpu_inc(*sem->read_count);
this_cpu_inc(*sem->read_count);
else
__percpu_down_read(sem, false); /* Unconditional memory barrier */
/*
@ -79,7 +79,7 @@ static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
* Same as in percpu_down_read().
*/
if (likely(rcu_sync_is_idle(&sem->rss)))
__this_cpu_inc(*sem->read_count);
this_cpu_inc(*sem->read_count);
else
ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
preempt_enable();
@ -103,7 +103,7 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
* Same as in percpu_down_read().
*/
if (likely(rcu_sync_is_idle(&sem->rss))) {
__this_cpu_dec(*sem->read_count);
this_cpu_dec(*sem->read_count);
} else {
/*
* slowpath; reader will only ever wake a single blocked
@ -115,7 +115,7 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
* aggregate zero, as that is the only time it matters) they
* will also see our critical section.
*/
__this_cpu_dec(*sem->read_count);
this_cpu_dec(*sem->read_count);
rcuwait_wake_up(&sem->writer);
}
preempt_enable();

4
kernel/locking/percpu-rwsem.c
View File

@ -45,7 +45,7 @@ EXPORT_SYMBOL_GPL(percpu_free_rwsem);
static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
__this_cpu_inc(*sem->read_count);
this_cpu_inc(*sem->read_count);
/*
* Due to having preemption disabled the decrement happens on
@ -71,7 +71,7 @@ static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
if (likely(!atomic_read_acquire(&sem->block)))
return true;
__this_cpu_dec(*sem->read_count);
this_cpu_dec(*sem->read_count);
/* Prod writer to re-evaluate readers_active_check() */
rcuwait_wake_up(&sem->writer);