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

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Message-ID: <CA+55aFzqzHa_6bHrqaa+6k3PZiibuJuDd1VwVD7jL047BjR=hA@mail.gmail.com>
Date:	Fri, 29 Mar 2013 20:46:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Emmanuel Benisty <benisty.e@...il.com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...riel.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>
Subject: Re: ipc,sem: sysv semaphore scalability

On Fri, Mar 29, 2013 at 8:02 PM, Emmanuel Benisty <benisty.e@...il.com> wrote:
>
> Then I start building a random package and the problems start. They
> may also happen without compiling but this seems to trigger the bug
> quite quickly.

I suspect it's about preemption, and the build just results in enough
scheduling load that you start hitting whatever race there is.

> Anyway, some progress here, I hope: dmesg seems to be
> willing to reveal some secrets (using some pastebin service since this
> is pretty big):
>
> https://gist.github.com/anonymous/5275120

That looks like exactly the exit_sem() bug that Davidlohr was talking
about, where the

                /* exit_sem raced with IPC_RMID, nothing to do */
                if (IS_ERR(sma))
                        continue;

should be moved to *before* the

                sem_lock(sma, NULL, -1);

call. And apparently the bug I had found is already fixed in -next.

               Linus
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