To: linux-kernel@vger.kernel.org
From: Sergei Organov <osv@javad.com>
Subject: Re: [PATCH] Document Linux's memory barriers [try #2]
Date: Thu, 09 Mar 2006 15:02:15 +0300
Message-ID: <dup5gf$t1t$1@sea.gmane.org>
References: <31492.1141753245@warthog.cambridge.redhat.com>
	<29826.1141828678@warthog.cambridge.redhat.com>
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David Howells <dhowells@redhat.com> writes:
[...]
> +=======================================
> +LINUX KERNEL COMPILER BARRIER FUNCTIONS
> +=======================================
> +
> +The Linux kernel has an explicit compiler barrier function that prevents the
> +compiler from moving the memory accesses either side of it to the other side:
> +
> +	barrier();
> +
> +This has no direct effect on the CPU, which may then reorder things however it
> +wishes.
> +
> +In addition, accesses to "volatile" memory locations and volatile asm
> +statements act as implicit compiler barriers.

This last statement seems to contradict with what GCC manual says about
volatile asm statements:

"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.*"

I think that volatile memory locations aren't compiler barriers either,
-- GCC only guarantees that it won't remove the access and that it won't
re-arrange the access w.r.t. other *volatile* accesses. On the other
hand, barrier() indeed prevents *any* memory access from being moved
across the barrier.

-- Sergei.

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