From: David Howells <dhowells@redhat.com>
In-Reply-To: <878xrecypp.fsf@javad.com> 
References: <878xrecypp.fsf@javad.com>  <16835.1141936162@warthog.cambridge.redhat.com> 
To: Sergei Organov <osv@javad.com>
Cc: David Howells <dhowells@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Document Linux's memory barriers [try #4] 
Date: Tue, 14 Mar 2006 20:31:57 +0000
Message-ID: <31016.1142368317@warthog.cambridge.redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1019
Lines: 24

Sergei Organov <osv@javad.com> wrote:

> "You can prevent an `asm' instruction from being deleted by writing the
> keyword `volatile' after the `asm'. [...]
> The `volatile' keyword indicates that the instruction has important
> side-effects.  GCC will not delete a volatile `asm' if it is reachable.
> (The instruction can still be deleted if GCC can prove that
> control-flow will never reach the location of the instruction.)  *Note
> that even a volatile `asm' instruction can be moved relative to other
> code, including across jump instructions.*"

Ummm... If "asm volatile" statements don't form compiler barriers, then how do
you specify a compiler barrier? Or is that what the "memory" bit in:

	#define barrier() __asm__ __volatile__("": : :"memory")

does?

David
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