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Date: Fri, 29 Mar 2013 16:16:01 -0700
Message-ID: <CA+55aFygTH-tqhL1=FY32+HkjfqqpkX-nvb1c5f3V6S-iykS8A@mail.gmail.com>
Subject: Re: ipc,sem: sysv semaphore scalability
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dave Jones <davej@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@surriel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        hhuang@redhat.com, "Low, Jason" <jason.low2@hp.com>,
        Michel Lespinasse <walken@google.com>,
        Larry Woodman <lwoodman@redhat.com>,
        "Vinod, Chegu" <chegu_vinod@hp.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Emmanuel Benisty <benisty.e@gmail.com>
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On Fri, Mar 29, 2013 at 2:12 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I dunno. I'm still not sure this is triggerable, but it looks bad. But
> both the semaphore case and the msg cases seem to be solvable by
> moving the unlock down, and shm seem to have no getref/putref users to
> race with, so this (whitespace-damaged) patch *may* be sufficient:

Well, the patch doesn't seem to cause any problems, at least neither
lockdep nor spinlock sleep debugging complains. I have no idea whether
it actually fixes any problems, though.

I do wonder if this might explain the problem Emmanuel saw. A double
free of a RCU-freeable object would possibly result in exactly the
kind of mess that Emmanuel reported with the semaphore scalability
patches.

Emmanuel, can you try the attached patch? I think it applies cleanly
on top of the scalability series too without any changes, but I didn't
check if the patches perhaps changed some of the naming or something.

              Linus

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