linux-kernel - Re: [PATCH] ipc/sem.c: Fix complex

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Date:	Mon, 4 Jan 2016 05:02:31 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
	Ingo Molnar <mingo@...hat.com>,
	felixh@...ormatik.uni-bremen.de, stable@...r.kernel.org
Subject: Re: [PATCH] ipc/sem.c: Fix complex_count vs. simple op race

On Sat, 02 Jan 2016, Manfred Spraul wrote:

>Commit 6d07b68ce16a ("ipc/sem.c: optimize sem_lock()") introduced a race:
>
>sem_lock has a fast path that allows parallel simple operations.
>There are two reasons why a simple operation cannot run in parallel:
>- a non-simple operations is ongoing (sma->sem_perm.lock held)
>- a complex operation is sleeping (sma->complex_count != 0)
>
>As both facts are stored independently, a thread can bypass the current
>checks by sleeping in the right positions. See below for more details
>(or kernel bugzilla 105651).
>
>The patch fixes that by creating one variable (complex_mode)
>that tracks both reasons why parallel operations are not possible.
>
>The patch also updates stale documentation regarding the locking.
>
>With regards to stable kernels:
>The patch is required for all kernels that include the commit 6d07b68ce16a
>("ipc/sem.c: optimize sem_lock()") (3.10?)
>
>The alternative is to revert the patch that introduced the race.

I am just now catching up with this, but a quick thought is that we probably
want to keep 6d07b68ce16a as waiting on unlocking all sem->lock should be
much worse for performance than keeping track of the complex 'mode'. Specially
if we have a large array.

Also, any idea what workload exposed this race? Anyway, will take a closer look
at the patch/issue.

Thanks,
Davidlohr
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