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

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Message-ID: <20712.1240502435@redhat.com>
Date:	Thu, 23 Apr 2009 17:00:35 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dhowells@...hat.com, Oleg Nesterov <oleg@...hat.com>,
	Trond.Myklebust@...app.com, serue@...ibm.com, steved@...hat.com,
	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slow_work_thread() should do the exclusive wait

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.  Bearing in mind
> that the thread might _already_ have been woken by someone else?

Perhaps the attached patch?

David
---
From: David Howells <dhowells@...hat.com>
Subject: [PATCH] slow_work_cull_timeout() should have a memory barrier

slow_work_cull_timeout() should have a write memory barrier so that the setting
of the cull flag is seen before the wakeup takes place.  This is required
because wake_up() does not guarantee any memory barriership at all.

Concomitant to this, slow_work_thread() should have a read memory barrier
between its return from schedule() and its testing of slow_work_cull() as
finish_wait() isn't a guaranteed barrier either.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 kernel/slow-work.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)


diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index 521ed20..96e418d 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -382,6 +382,7 @@ static int slow_work_thread(void *_data)
 		finish_wait(&slow_work_thread_wq, &wait);
 
 		try_to_freeze();
+		smp_rmb();
 
 		vsmax = vslow_work_proportion;
 		vsmax *= atomic_read(&slow_work_thread_count);
@@ -416,6 +417,7 @@ static int slow_work_thread(void *_data)
 static void slow_work_cull_timeout(unsigned long data)
 {
 	slow_work_cull = true;
+	smp_wmb();
 	wake_up(&slow_work_thread_wq);
 }
 
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