linux-kernel - Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

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Date:	Fri, 22 Feb 2013 15:05:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Kevin Hilman <khilman@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:
> > Which is a problem how?
> 
> So here is a possible scenario, CPU 0 reads a kcpustat value, and CPU
> 1 writes
> it at the same time:
> 
>     //Initial value of "cpustat" is 0xffffffff
>          == CPU 0 ==           == CPU 1 ==
> 
>        //load low part
>        mov %eax, [cpustat]
>                              inc [cpustat]
>                              //Update the high part if necessary
>                              jnc 1f
>                              inc [cpustat + 4]
>                              1:
>        //load high part
>        mov %edx, [cpustat + 4]
> 
> 
> Afterward, CPU 0 will think the value is 0x1ffffffff while it's
> actually
> 0x100000000.
> 
> atomic64_read() and atomic64_set() are supposed to take care of that,
> without
> even the need for _inc() or _add() parts that use LOCK.


Sure I get that, but again, why is that a problem,.. who relies on
these statistics that makes it a problem?

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