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Date: Mon, 16 Feb 2009 02:38:12 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Li Zefan <lizf@cn.fujitsu.com>
Cc: containers@lists.osdl.org, LKML <linux-kernel@vger.kernel.org>,
       Paul Menage <menage@google.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Arjan van de Ven <arjan@infradead.org>, gregkh@ZenIV.linux.org.uk,
       Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636
	mntput_no_expire+0xac/0xf2()
Message-ID: <20090216023812.GT28946@ZenIV.linux.org.uk>
References: <4993C2A0.3050507@cn.fujitsu.com> <4993C7C2.4060100@cn.fujitsu.com> <20090212070729.GF28946@ZenIV.linux.org.uk> <4995007D.7040101@cn.fujitsu.com> <20090213054751.GI28946@ZenIV.linux.org.uk> <49950F3D.3030704@cn.fujitsu.com> <20090213064135.GJ28946@ZenIV.linux.org.uk> <20090213071816.GK28946@ZenIV.linux.org.uk> <4995208D.8050700@cn.fujitsu.com> <4998C15C.2070203@cn.fujitsu.com>
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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;
 		/*