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

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Message-ID: <530A76F7.3010708@hurleysoftware.com>
Date:	Sun, 23 Feb 2014 17:32:23 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	Tejun Heo <tj@...nel.org>, Lai Jiangshan <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

Hi James,

On 02/23/2014 03:05 PM, James Bottomley wrote:
> 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.

AFAIK, Paul will not be changing the quoted text above.

> 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.

The reason why there is no deadlock here is because the RELEASE M is
not dependent on the ACQUIRE N to complete.

If the attempt to ACQUIRE N is speculated before the RELEASE M, two
possibilities exist:
1. N is not owned, so the ACQUIRE is immediately successful, or
2. N is owned, so the attempted ACQUIRE is not immediately successful.

However, in both cases the RELEASE M will still complete, having already
been started (since it occurred before in the instruction stream).

Regards,
Peter Hurley




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