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From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Ingo Molnar <mingo@kernel.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Jason Low <jason.low2@hp.com>,
	Michel Lespinasse <walken@google.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks
Date: Tue, 27 Jan 2015 18:07:32 +0100	[thread overview]
Message-ID: <20150127170732.GI21418@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1422257769-14083-3-git-send-email-dave@stgolabs.net>

On Sun, Jan 25, 2015 at 11:36:05PM -0800, Davidlohr Bueso wrote:
> The need for the smp_mb in __rwsem_do_wake should be
> properly documented. Applies to both xadd and spinlock
> variants.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
>  kernel/locking/rwsem-spinlock.c | 5 +++++
>  kernel/locking/rwsem-xadd.c     | 5 +++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
> index 2555ae1..54f7a17 100644
> --- a/kernel/locking/rwsem-spinlock.c
> +++ b/kernel/locking/rwsem-spinlock.c
> @@ -85,6 +85,11 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
>  
>  		list_del(&waiter->list);
>  		tsk = waiter->task;
> +		/*
> +		 * Ensure that all cores see the read before
> +		 * setting it to the waiter task to nil, as that
> +		 * grants the read lock to the next task.
> +		 */
>  		smp_mb();
>  		waiter->task = NULL;
>  		wake_up_process(tsk);

Could you enhance that comment by pointing at the pairing code? Is that
the wait loop in rwsem_down_read_failed()?

Also, the comment confuses, how can all cores observe a read into a
local variable?

  reply	other threads:[~2015-01-27 17:07 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-26  7:36 [PATCH -tip 0/6] rwsem: Fine tuning Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 1/6] locking/rwsem: Use task->state helpers Davidlohr Bueso
2015-02-04 14:38   ` [tip:locking/core] " tip-bot for Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 2/6] locking/rwsem: Document barrier need when waking tasks Davidlohr Bueso
2015-01-27 17:07   ` Peter Zijlstra [this message]
2015-01-27 20:30     ` Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 3/6] locking/rwsem: Set lock ownership ASAP Davidlohr Bueso
2015-01-27 17:10   ` Peter Zijlstra
2015-01-27 19:18     ` Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 4/6] locking/rwsem: Avoid deceiving lock spinners Davidlohr Bueso
2015-01-27 17:23   ` Jason Low
2015-01-28  3:54     ` Davidlohr Bueso
2015-01-28 17:01       ` Tim Chen
2015-01-28 21:03       ` Jason Low
2015-01-29  1:10         ` Davidlohr Bueso
2015-01-29 20:13           ` Jason Low
2015-01-29 20:18             ` Jason Low
2015-01-29 23:15               ` Davidlohr Bueso
2015-01-30  1:52                 ` Refactoring mutex spin on owner code Jason Low
2015-01-30  7:14                   ` Davidlohr Bueso
2015-01-30  7:51                     ` Peter Zijlstra
2015-01-26  7:36 ` [PATCH 5/6] locking/rwsem: Optimize slowpath/sleeping Davidlohr Bueso
2015-01-27 17:34   ` Peter Zijlstra
2015-01-27 21:57     ` Davidlohr Bueso
2015-01-26  7:36 ` [PATCH 6/6] locking/rwsem: Check for active lock before bailing on spinning Davidlohr Bueso
2015-01-27 18:11   ` Jason Low

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