Date: Sat, 13 Dec 2014 23:39:18 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Jones <davej@redhat.com>, Chris Mason <clm@fb.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        =?iso-8859-1?Q?D=E2niel?= Fraga <fragabr@gmail.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: frequent lockups in 3.18rc4
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On Sat, Dec 13, 2014 at 02:59:43PM -0800, Linus Torvalds wrote:
> Side note: I think I've found a real potential lockup bug in
> fs/namespace.c, but afaik it could only trigger with the RT patches.

> Except it won't with the RT patches, I guess. So it looks like you could have:\
> 
>  - mnt_make_readonly() sets that bit
>  - gets preempted with the RT patches
>  - we run mnt_want_write() on all CPU's, which disables preemption and
> waits for the bit to be cleared
>  - nothing happens.
> 
> This is clearly not what happens in your lockup, but it does seem to
> be a potential issue for the RT kernel.
> 
> Added Al and Thomas to the cc, for fs/namespace.c and RT kernel
> respectively. Maybe the RT patches already fix this, I didn't actually
> check.

I agree that it's a thing to keep in mind on the RT side of things, but
IMO it belongs in RT patches...
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