linux-kernel - Re: [PATCH] slow_work_thread() should do the exclusive wait

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Message-ID: <20090416143351.GD6532@redhat.com>
Date:	Thu, 16 Apr 2009 16:33:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Trond.Myklebust@...app.com, serue@...ibm.com, steved@...hat.com,
	viro@...iv.linux.org.uk, Daire.Byrne@...mestore.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slow_work_thread() should do the exclusive wait

On 04/16, David Howells wrote:
>
> Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> > I wonder if slow_work_cull_timeout() should have some sort of barrier,
> > so the write is suitably visible to the woken thread.
>
> That's an interesting question.  Should wake_up() imply a barrier of any sort,
> I wonder.  Well, __wake_up() does impose a barrier as it uses a spinlock, but
> I wonder if that's sufficient.

wake_up() does imply the barrier. Note the smp_wmb() in try_to_wake_up().
And in fact this wmb() implies mb(), because spin_lock() itself is STORE,
and the futher LOADs can't leak up before spin_lock().

But afaics, this doesn't matter? prepare_to_wait() sets task->state under
wait_queue_head_t->lock and wake_up() takes this look too, so we can't miss
the event.

Or I completely misunderstood the issue...

Oleg.

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