From: Oleg Nesterov <oleg@redhat.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: mingo@kernel.org, peterz@infradead.org, john.stultz@linaro.org,
dimitrysh@google.com, linux-kernel@vger.kernel.org,
Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups
Date: Mon, 21 Nov 2016 13:23:44 +0100 [thread overview]
Message-ID: <20161121122343.GA635@redhat.com> (raw)
In-Reply-To: <1479495277-9075-4-git-send-email-dave@stgolabs.net>
On 11/18, Davidlohr Bueso wrote:
>
> +static bool __readers_active_check(struct percpu_rw_semaphore *sem)
> +{
> + return !(per_cpu_sum(*sem->read_count) !=0);
> +}
Hmm,
return per_cpu_sum(*sem->read_count) == 0;
looks more clear, but this is minor,
> int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
> const char *name, struct lock_class_key *rwsem_key)
> {
> @@ -103,41 +141,11 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
> __this_cpu_dec(*sem->read_count);
>
> /* Prod writer to recheck readers_active */
> - swake_up(&sem->writer);
> + if (__readers_active_check(sem))
> + swake_up(&sem->writer);
Suppose we have 2 active readers which call __percpu_up_read() at the same
time and the pending writer sleeps.
What guarantees that one of these readers will observe per_cpu_sum() == 0 ?
They both can read the old value of the remote per-cpu counter, no?
Oleg.
next prev parent reply other threads:[~2016-11-21 12:23 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-18 18:54 [PATCH -tip 0/3] locking/percpu-rwsem: writer-side optimizations Davidlohr Bueso
2016-11-18 18:54 ` [PATCH 1/3] locking/percpu-rwsem: Move text file into Documentation/locking/ Davidlohr Bueso
2016-11-18 18:54 ` [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait Davidlohr Bueso
2016-11-21 12:55 ` Oleg Nesterov
2016-11-21 17:26 ` Davidlohr Bueso
2016-12-03 2:18 ` [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues Davidlohr Bueso
2016-12-05 8:36 ` Peter Zijlstra
2016-12-05 11:26 ` Oleg Nesterov
2016-12-05 11:32 ` Oleg Nesterov
2016-12-05 17:37 ` Davidlohr Bueso
2016-12-05 17:19 ` Oleg Nesterov
2016-12-05 17:13 ` Oleg Nesterov
2016-11-18 18:54 ` [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Davidlohr Bueso
2016-11-21 12:23 ` Oleg Nesterov [this message]
2016-11-21 12:29 ` Peter Zijlstra
2016-11-21 12:47 ` Oleg Nesterov
2016-11-21 15:07 ` Oleg Nesterov
2016-11-22 3:59 ` Davidlohr Bueso
2016-11-23 14:43 ` Oleg Nesterov
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