linux-kernel - Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED

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Message-ID: <1393185948.9743.2.camel@dabdike>
Date:	Sun, 23 Feb 2014 14:05:48 -0600
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Tejun Heo <tj@...nel.org>, laijs@...fujitsu.com,
	linux-kernel@...r.kernel.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux1394-devel@...ts.sourceforge.net,
	Chris Boot <bootc@...tc.net>, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org
Subject: Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK

On Sat, 2014-02-22 at 14:03 -0500, Peter Hurley wrote:
> If it is necessary for a RELEASE-ACQUIRE pair to produce a full barrier, the
> ACQUIRE can be followed by an smp_mb__after_unlock_lock() invocation.  This
> will produce a full barrier if either (a) the RELEASE and the ACQUIRE are
> executed by the same CPU or task, or (b) the RELEASE and ACQUIRE act on the
> same variable.  The smp_mb__after_unlock_lock() primitive is free on many
> architectures.  Without smp_mb__after_unlock_lock(), the critical sections
> corresponding to the RELEASE and the ACQUIRE can cross:
> 
> 	*A = a;
> 	RELEASE M
> 	ACQUIRE N
> 	*B = b;
> 
> could occur as:
> 
> 	ACQUIRE N, STORE *B, STORE *A, RELEASE M

Ah, OK, that's an error in the documentation.  The example should read

	*A = a;
 	RELEASE *N*
 	ACQUIRE *M*
 	*B = b;

The point being you can't have speculation that entangles critical
sections, as I've been saying, because that would speculate you into
ABBA deadlocks.  Paul McKenny will submit a patch fixing the bug in
documentation.

James


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