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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