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

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Date:	Fri, 13 Feb 2009 15:26:05 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	containers@...ts.osdl.org, Paul Menage <menage@...gle.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636	mntput_no_expire+0xac/0xf2()

Al Viro wrote:
> On Fri, Feb 13, 2009 at 06:41:35AM +0000, Al Viro wrote:
> 
> Aaaargh...
> 
>         /*
>          * We don't have to hold all of the locks at the
>          * same time here because we know that we're the
>          * last reference to mnt and that no new writers
>          * can come in.
>          */
>         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);
> 
> is *almost* OK.  Modulo SMP cache coherency.  We know that nothing should
> be setting ->mnt to ours anymore, that's fine.  But we do not know if
> we'd seen *earlier* change done on CPU in question (not the one we
> are running __mntput() on).
> 
> I probably would still like to use milder solution in the long run, but for
> now let's check if turning that into
> 
>                 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. :)

BTW, thread2's rmdir failed:

rmdir: /cgroup/0: No such file or directory

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