From mboxrd@z Thu Jan  1 00:00:00 1970
From: peterz@infradead.org (Peter Zijlstra)
Date: Fri, 22 Feb 2013 15:05:39 +0100
Subject: [PATCH 0/2] cpustat: use atomic operations to read/update stats
In-Reply-To: <20130222125019.GC17948@somewhere.redhat.com>
References: <1361512604-2720-1-git-send-email-khilman@linaro.org>
 <1361522767.26780.44.camel@laptop>
 <20130222125019.GC17948@somewhere.redhat.com>
Message-ID: <1361541939.26780.63.camel@laptop>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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?