linux-kernel - Re: Broken ARM atomic ops wrt memory barriers (was : [PATCH] Add cmpxchg support for ARMv6+ systems)

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Message-ID: <20090527085631.GA18768@n2100.arm.linux.org.uk>
Date:	Wed, 27 May 2009 09:56:31 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jamie Lokier <jamie@...reable.org>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Broken ARM atomic ops wrt memory barriers (was : [PATCH] Add
	cmpxchg support for ARMv6+ systems)

On Tue, May 26, 2009 at 09:22:43PM -0400, Mathieu Desnoyers wrote:
> My point was not about the wmb() : we _clearly_ need a dmb there. My
> point is about the hidden
> 
> smp_read_barrier_depends() in rcu_dereference() :
> 
> cpyptr = ptr;
> smp_read_barrier_depends();
> access *cpyptr data
> 
> Which is needed to make sure we update our global view of memory between
> the pointer value read and the moment we read the data pointed to. This
> makes sure the data read is not garbage.

There is a dependency between reading 'ptr' and reading 'data'.
I think there was a paragraph in the ARM ARM about this which says
that they happen in program order, but I don't have access at the
moment to the ARM ARM to check right now.
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