linux-kernel - Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636 mntput_no

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Date:	Mon, 16 Feb 2009 02:38:12 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	containers@...ts.osdl.org, LKML <linux-kernel@...r.kernel.org>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	gregkh@...IV.linux.org.uk,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636
	mntput_no_expire+0xac/0xf2()

On Mon, Feb 16, 2009 at 09:29:00AM +0800, Li Zefan wrote:
> >>                 struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
> >>                 spin_lock(&cpu_writer->lock);
> >>                 if (cpu_writer->mnt != mnt) {
> >> 			spin_unlock(&cpu_writer->lock);
> >>                         continue;
> >> 		}
> >> prevents the problem, OK?
> >>
> > 
> > Sure, I'll try. :)
> > 
> 
> Not a single warning for the whole weekend, so I think above change works.

OK...  So here's what we really want:
	* we know that nobody will set cpu_writer->mnt to mnt from now on
	* all changes to that sucker are done under cpu_writer->lock
	* we want the laziest equivalent of
		spin_lock(&cpu_writer->lock);
		if (likely(cpu_writer->mnt != mnt)) {
			spin_unlock(&cpu_writer->lock);
			continue;
		}
		/* do stuff */
that would make sure we won't miss earlier setting of ->mnt done by another
CPU.

Anyway, for now (HEAD and all -stable starting with 2.6.26) we want this:

--- fs/namespace.c	2009-01-25 21:45:31.000000000 -0500
+++ fs/namespace.c	2009-02-15 21:31:14.000000000 -0500
@@ -614,9 +614,11 @@
 	 */
 	for_each_possible_cpu(cpu) {
 		struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
-		if (cpu_writer->mnt != mnt)
-			continue;
 		spin_lock(&cpu_writer->lock);
+		if (cpu_writer->mnt != mnt) {
+			spin_unlock(&cpu_writer->lock);
+			continue;
+		}
 		atomic_add(cpu_writer->count, &mnt->__mnt_writers);
 		cpu_writer->count = 0;
 		/*
--
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