linux-kernel - Re: [PATCH] bitops: add

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Message-ID: <4FAA9A49.8080900@zytor.com>
Date:	Wed, 09 May 2012 09:24:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rob Landley <rob@...dley.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Akinobu Mita <akinobu.mita@...il.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, Gleb Natapov <gleb@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] bitops: add _local bitops

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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