From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Nicholas Mc Guire <der.herr@hofr.at>
Cc: linux-rt-users@vger.kernel.org,
LKML <linux-kernel@vger.kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
Peter Zijlstra <peterz@infradead.org>,
Carsten Emde <C.Emde@osadl.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andreas Platschek <platschek@ict.tuwien.ac.at>
Subject: Re: [PATCH RT 2/5] allow preemption in alloc and free _buffer_head
Date: Fri, 14 Feb 2014 13:53:25 +0100 [thread overview]
Message-ID: <20140214125325.GD28438@linutronix.de> (raw)
In-Reply-To: <20140210153759.GC20017@opentech.at>
* Nicholas Mc Guire | 2014-02-10 16:37:59 [+0100]:
>2a) premption induced race in __this_cpu_dec
> T1 dec:start
> T2 dec
> T1 dec:end
>
>__this_cpu_dec
> -> __this_cpu_sub
> -> __this_cpu_add
> -> __this_cpu_add_#
> -> __this_cpu_generic_to_op
> -> __this_cpu_ptr += val
>
>
>so we end up with the possibilities of
> T1 load
> T2 load
> T2 store
> T1 store
>and the increment provided by T2 would be lost. The result of which
>is that reaching the buffer_heads_over_limit threshold in recalc_bh_state
I don't like this very much. __this_cpu_inc() is not atomic as you say
and without the locking that is provided by preempt_disable() it might
lose one store in inc or dec direction. This error will never be
corrected again. That code path looks very short unless you have 4096
CPUs.
Sebastian
prev parent reply other threads:[~2014-02-14 12:53 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-10 15:37 [PATCH RT 2/5] allow preemption in alloc and free _buffer_head Nicholas Mc Guire
2014-02-14 12:53 ` Sebastian Andrzej Siewior [this message]
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