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From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
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Date: Tue, 22 Nov 2011 13:57:42 +0100
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On Tue, 2011-11-22 at 13:21 +0100, Jan Kara wrote:
> > +             __get_cpu_var(bdp_ratelimits)++;
>   I think you need preempt_disable() and preempt_enable() pair around
> __get_cpu_var(). Otherwise a process could get rescheduled in the middle of
> read-modify-write cycle... 

there's of course the this_cpu_inc(bdp_ratelimits); thing.

On x86 that'll turn into a single insn, on others it will add the
required preempt_disable/enable bits.