Message-ID: <4FAA9A49.8080900@zytor.com>
Date: Wed, 09 May 2012 09:24:41 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: "Michael S. Tsirkin" <mst@redhat.com>
CC: Rob Landley <rob@landley.net>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Howells <dhowells@redhat.com>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Herbert Xu <herbert@gondor.hengli.com.au>,
        Stephen Rothwell <sfr@canb.auug.org.au>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        Gleb Natapov <gleb@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
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        Marcelo Tosatti <mtosatti@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] bitops: add _local bitops
References: <20120509134528.GA18044@redhat.com> <4FAA7939.6040706@zytor.com> <20120509154734.GB20867@redhat.com>
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On 05/09/2012 08:47 AM, Michael S. Tsirkin wrote:
> 
> By the way, clear_bit on x86 does not seem to contain
> an optimization barrier - is my reading correct?
> Lock prefix does not affect the compiler, right?

Yes, as it clearly states in the comment:

 * clear_bit() is atomic and may not be reordered.  However, it does
 * not contain a memory barrier, so if it is used for locking purposes,
 * you should call smp_mb__before_clear_bit() and/or
smp_mb__after_clear_bit()
 * in order to ensure changes are visible on other processors.

There is clear_bit_unlock() which has the barrier semantics.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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