linux-kernel - Re: ipc,sem: sysv semaphore scalability

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Date:	Fri, 29 Mar 2013 16:16:01 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...riel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	hhuang@...hat.com, "Low, Jason" <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Larry Woodman <lwoodman@...hat.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	Peter Hurley <peter@...leysoftware.com>,
	Emmanuel Benisty <benisty.e@...il.com>
Subject: Re: ipc,sem: sysv semaphore scalability

On Fri, Mar 29, 2013 at 2:12 PM, Linus Torvalds
<torvalds@...ux-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

Download attachment "patch.diff" of type "application/octet-stream" (1207 bytes)