Re: [RFC][PATCH 5/5] percpu-rwsem: Optimize readers and reduce global impact
From: Peter Zijlstra
Date: Tue Jun 16 2015 - 07:48:38 EST
On Sat, May 30, 2015 at 10:49:00PM +0200, Oleg Nesterov wrote:
> > On Fri, May 29, 2015 at 1:09 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > >
> > > Doesn't it need mb() before "state = readers_slow" to ensure
> > > "release" semantics?
> __percpu_down_read() lacks another mb() after the "state != BLOCK"
> check for the same reason, and we can use smp_load_acquire(state)
I made the below modification to the patch.
@@ -51,7 +51,11 @@ void __percpu_down_read(struct percpu_rw
smp_mb(); /* A matches D */
- if (likely(sem->state != readers_block))
+ * If !readers_block the critical section starts here, matched by the
+ * release in percpu_up_write().
+ if (likely(smp_load_acquire(sem->state) != readers_block))
@@ -154,8 +158,11 @@ void percpu_up_write(struct percpu_rw_se
* One reason that we cannot just immediately flip to readers_fast is
* that new readers might fail to see the results of this writer's
* critical section.
+ * Therefore we force it through the slow path which guarantees an
+ * acquire and thereby guarantees the critical section's consistency.
- sem->state = readers_slow;
+ smp_store_release(sem->state, readers_slow);
* Release the write lock, this will allow readers back in the game.
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