Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

[peterz]

On Tue, Sep 15, 2020 at 05:51:50PM +0200, pet...@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 <hout...@huawei.com>
Date: Tue, 15 Sep 2020 22:07:50 +0800

From: Hou Tao <hout...@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 <hout...@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Link: https://lkml.kernel.org/r/20200915140750.137881-1-hout...@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);