linux-kernel - Re: ARM: why smp_mb() is not needed in the "__mutex_fastpath_lock" and "__mutex_fastpath

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Message-ID: <20120713094232.GD18079@mudshark.cambridge.arm.com>
Date:	Fri, 13 Jul 2012 10:42:32 +0100
From:	Will Deacon <will.deacon@....com>
To:	shan kang <kangshan0910@...il.com>
Cc:	Li Haifeng <omycle@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: ARM: why smp_mb() is not needed in the "__mutex_fastpath_lock"
 and "__mutex_fastpath_unlock" functions

On Fri, Jul 13, 2012 at 10:10:52AM +0100, shan kang wrote:
> For example, in the following scenario, Process2  may get the wrong value;
> Process1:
> mutex_lock(&lock);
> write data; (store operation)
> mutex_unlock(&lock);
> 
> Process2:
> mutex_lock(&lock);
> read data; (load operation)
> mutex_unlock(&lock);

Yes, it looks like we can screw things up in the uncontended case (where
nobody blocks on the mutex). We could add an smp_mb after the lock operation
and another one before the unlock, but I'm tempted just to use
asm-generic/mutex-dec.h instead. The latter approach will subtly change the
current behaviour, so I'll post a patch when I'm happy with it.

Curious: did you find this by inspection or did you observe it going wrong?

Cheers,

Will
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