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SubjectRe: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count
On Tue, Sep 15, 2020 at 05:51:50PM +0200, peterz@infradead.org wrote:

> Anyway, I'll rewrite the Changelog and stuff it in locking/urgent.

How's this?

---
Subject: locking/percpu-rwsem: Use this_cpu_{inc,dec}() for read_count
From: Hou Tao <houtao1@huawei.com>
Date: Tue, 15 Sep 2020 22:07:50 +0800

From: Hou Tao <houtao1@huawei.com>

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.

Fixing 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: 70fe2f48152e ("aio: fix freeze protection of aio writes")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200915140750.137881-1-houtao1@huawei.com
---
include/linux/percpu-rwsem.h | 8 ++++----
kernel/locking/percpu-rwsem.c | 4 ++--
2 files changed, 6 insertions(+), 6 deletions(-)

--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -60,7 +60,7 @@ static inline void percpu_down_read(stru
* 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_tryl
* 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
* 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
* 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();
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -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(s
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);
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