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Subject: RE: [BUG] Variable stopmachine_state should be volatile
Date: Thu, 1 Dec 2005 10:58:33 +0800
Message-ID: <8126E4F969BA254AB43EA03C59F44E84040B3C8C@pdsmsx404>
Thread-Topic: [BUG] Variable stopmachine_state should be volatile
Thread-Index: AcX1/DPw2jHjzQdxR9aiZehA42spSQAJpdpA
From: "Zhang, Yanmin" <yanmin.zhang@intel.com>
To: "Pavel Machek" <pavel@ucw.cz>
Cc: <linux-kernel@vger.kernel.org>,
       "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>,
       "Shah, Rajesh" <rajesh.shah@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1187
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>>-----Original Message-----
>>From: Pavel Machek [mailto:pavel@ucw.cz]
>>Sent: 2005??12??1?? 6:20
>>To: Zhang, Yanmin
>>Cc: linux-kernel@vger.kernel.org; Pallipadi, Venkatesh; Shah, Rajesh
>>Subject: Re: [BUG] Variable stopmachine_state should be volatile
>>
>>Hi!
>>
>>> The model to access variable stopmachine_state is that a main thread
>>> writes it and other threads read it. Its declaration has no sign
>>> volatile. In the while loop in function stopmachine, this variable is
>>> read, and compiler might optimize it by reading it once before the loop
>>> and not reading it again in the loop, so the thread might enter dead
>>> loop.
>>
>>No. volatile may look like a solution, but it usually is not. You may
>>need some barriers, atomic_t or locking.
>>								Pavel
The original functions already use smp_mb/smp_wmb. My patch just tells compiler not to optimize by bringing the reading of stopmachine_state out of the while loop.

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