linux-kernel - Re: [PATCH 1/2] ipc/sem.c: Fix complex

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Date:	Mon, 20 Jun 2016 16:04:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-next@...r.kernel.org,
	1vier1@....de, Davidlohr Bueso <dave@...olabs.net>,
	felixh@...ormatik.uni-bremen.de, <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race

On Sat, 18 Jun 2016 22:02:21 +0200 Manfred Spraul <manfred@...orfullife.com> 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?)

I've had this in -mm (and -next) since January 4, without issues.  I
put it on hold because Davidlohr expressed concern about performance
regressions.

Your [2/2] should prevent those regressions (yes?) so I assume that any
kernel which has [1/2] really should have [2/2] as well.  But without
any quantitative information, this is all mad guesswork.

What to do?

(The [2/2] changelog should explain that it is the cure to [1/2]'s
regressions, btw).