Message-ID: <1393185948.9743.2.camel@dabdike>
Subject: Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Tejun Heo <tj@kernel.org>, laijs@cn.fujitsu.com,
        linux-kernel@vger.kernel.org,
        Stefan Richter <stefanr@s5r6.in-berlin.de>,
        linux1394-devel@lists.sourceforge.net, Chris Boot <bootc@bootc.net>,
        linux-scsi@vger.kernel.org, target-devel@vger.kernel.org
Date: Sun, 23 Feb 2014 14:05:48 -0600
In-Reply-To: <5308F48C.8080609@hurleysoftware.com>
References: <1392929071-16555-5-git-send-email-tj@kernel.org>
			 <5306AF8E.3080006@hurleysoftware.com>
			 <20140221015935.GF6897@htj.dyndns.org>
			 <5306B4DF.4000901@hurleysoftware.com>
			 <20140221021341.GG6897@htj.dyndns.org>
			 <5306E06C.5020805@hurleysoftware.com>
			 <20140221100301.GA14653@mtj.dyndns.org>
			 <53074BE4.1020307@hurleysoftware.com>
			 <20140221130614.GH6897@htj.dyndns.org>
			 <5307849A.9050209@hurleysoftware.com>
			 <20140221165730.GA10929@htj.dyndns.org>
			 <5307DAC9.2020103@hurleysoftware.com>
		 <1393094608.11497.1.camel@dabdike.int.hansenpartnership.com>
		 <5308F0E2.3030804@hurleysoftware.com>
	 <1393095138.11497.5.camel@dabdike.int.hansenpartnership.com>
	 <5308F48C.8080609@hurleysoftware.com>
Content-Type: text/plain; charset="ISO-8859-15"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org

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


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/