linux-kernel - Re: sequence lock in Linux

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Message-ID: <4C12BB7C.1040408@zytor.com>
Date:	Fri, 11 Jun 2010 15:41:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org
Subject: Re: sequence lock in Linux

On 06/11/2010 03:04 PM, Paul E. McKenney wrote:
> On Fri, Jun 11, 2010 at 02:38:59PM -0700, H. Peter Anvin wrote:
>> On 06/11/2010 02:36 PM, Paul E. McKenney wrote:
>>> Memory barriers in the sequence-lock code prevent this, assuming, as
>>> you point out, that memory clobber works (but if it doesn't, it should
>>> be fixed):
>>
>> The constness is my main concern.  It's not clear to me that "memory" is
>> meant to imply that const memory areas without volatile can be clobbered.
> 
> Ah!  I was assuming that gcc treated "memory" as it would an call to
> a function in some other compilation unit.  In that case, the compiler
> could not count on the "const" on the argument, given the possibility
> that the called function might gain a reference to the same memory
> locations in a non-const manner, right?
> 
> 							Thanx, Paul

Right.

	-hpa

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