From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Fri, 13 Jul 2012 10:42:32 +0100 Subject: ARM: why smp_mb() is not needed in the "__mutex_fastpath_lock" and "__mutex_fastpath_unlock" functions In-Reply-To: References: Message-ID: <20120713094232.GD18079@mudshark.cambridge.arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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