linux-kernel - Re: [PATCH] shm: optimize exit

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Message-ID: <20110803191639.GA6306@albatros>
Date:	Wed, 3 Aug 2011 23:16:39 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Manuel Lauss <manuel.lauss@...glemail.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Richard Weinberger <richard@....at>,
	Marc Zyngier <marc.zyngier@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shm: optimize exit_shm()

Hi Manuel,

On Wed, Aug 03, 2011 at 21:08 +0200, Manuel Lauss wrote:
> On Wed, Aug 3, 2011 at 8:28 PM, Vasiliy Kulikov <segoon@...nwall.com> wrote:
> > We may check .in_use == 0 without holding the rw_mutex as .in_use is int
> > and reads of ints are atomic.  As .in_use may be changed to zero while current
> > process was sleeping in down_write(), we should check .in_use once again after
> > down_write().
[...]
> > +       if (shm_ids(ns).in_use == 0)
> > +               return;
> > +
> >        /* Destroy all already created segments, but not mapped yet */
> >        down_write(&shm_ids(ns).rw_mutex);
> >        if (shm_ids(ns).in_use)
> 
> This check here is now unnecessary, yes?

No, as I said in the comment above, other task may be holding the mutex and
deleting the last shm segment.  So, current task will see in_use == 1
before down_write(), but == 0 after it.

> And this also fixes the oops.

Yes, but it only hides the real problem - tasks' dependency on initialized
init_*_ns.

Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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