linux-kernel - Re: [PATCH RT 2/5] allow preemption in alloc and free _buffer

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Date:	Fri, 14 Feb 2014 13:53:25 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	linux-rt-users@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Platschek <platschek@....tuwien.ac.at>
Subject: Re: [PATCH RT 2/5] allow preemption in alloc and free _buffer_head

* 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
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