Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK

[Peter Hurley]

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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