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

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Message-ID: <20110803194344.GA3160@redhat.com>
Date:	Wed, 3 Aug 2011 21:43:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Vasiliy Kulikov <segoon@...nwall.com>
Cc:	Manuel Lauss <manuel.lauss@...glemail.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Richard Weinberger <richard@....at>,
	Marc Zyngier <marc.zyngier@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] shm: optimize exit_shm()

On 08/03, Vasiliy Kulikov wrote:
>
> On Wed, Aug 03, 2011 at 21:29 +0200, Oleg Nesterov wrote:
> > On 08/03, Vasiliy Kulikov wrote:
> > >
> > > On Wed, Aug 03, 2011 at 21:08 +0200, Manuel Lauss wrote:
> > > > > +
> > > > >        /* 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? Why we can not do idr_for_each() in this (unikely) case?
>
> Because it's pointless.  idr_for_each() would not find any used segment.

This is clear. But it seems that me + Manuel were equally confused
by the changelog.

> > > > And this also fixes the oops.
> > >
> > > Yes, but it only hides the real problem - tasks' dependency on initialized
> > > init_*_ns.
> >
> > This is true, but your patch has the same dependency, but pretends
> > it doesn't ;) and it complicates the code.
>
> I didn't say that .in_use check fixes the oops.

I meant your shm-fix-a-race-between-shm_exit-and-shm_init.patch
which should be dropped imho ;)

Oleg.

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