From: Andrew Morton <akpm@linux-foundation.org>
To: Manfred Spraul <manfred@colorfullife.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
"H. Peter Anvin" <hpa@zytor.com>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-next@vger.kernel.org, 1vier1@web.de,
Davidlohr Bueso <dave@stgolabs.net>,
felixh@informatik.uni-bremen.de, <stable@vger.kernel.org>
Subject: Re: [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race
Date: Mon, 20 Jun 2016 16:04:56 -0700 [thread overview]
Message-ID: <20160620160456.a07982236e08d6d6be4cd442@linux-foundation.org> (raw)
In-Reply-To: <1466280142-19741-1-git-send-email-manfred@colorfullife.com>
On Sat, 18 Jun 2016 22:02:21 +0200 Manfred Spraul <manfred@colorfullife.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).
next prev parent reply other threads:[~2016-06-20 23:05 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-15 5:23 linux-next: manual merge of the akpm-current tree with the tip tree Stephen Rothwell
2016-06-18 19:39 ` Manfred Spraul
2016-06-18 20:02 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-06-18 20:02 ` [PATCH 2/2] ipc/sem: sem_lock with hysteresis Manfred Spraul
2016-06-21 20:29 ` Davidlohr Bueso
2016-06-25 17:37 ` Manfred Spraul
2016-06-28 17:54 ` Davidlohr Bueso
2016-06-20 23:04 ` Andrew Morton [this message]
2016-06-23 18:55 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-06-21 0:30 ` Davidlohr Bueso
2016-06-23 19:22 ` Manfred Spraul
2016-06-28 5:27 ` Davidlohr Bueso
2016-06-30 19:28 ` Manfred Spraul
2016-07-01 16:52 ` Davidlohr Bueso
2016-06-25 17:37 [PATCH 0/2] ipc/sem.c: sem_lock fixes Manfred Spraul
2016-06-25 17:37 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-07-13 5:06 [PATCH 0/2] ipc/sem.c: sem_lock fixes Manfred Spraul
2016-07-13 5:06 ` [PATCH 1/2] ipc/sem.c: Fix complex_count vs. simple op race Manfred Spraul
2016-07-16 1:27 ` Davidlohr Bueso
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