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SubjectRe: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK
On Tue, Dec 10, 2013 at 09:12:47AM -0800, Paul E. McKenney wrote:
> Good point -- the UNLOCK and LOCK are guaranteed to be ordered only
> if they both operate on the same lock variable. OK, I will make the
> example use different lock variables and show the different outcomes.
> How about the following?
>
> If it is necessary for an UNLOCK-LOCK pair to
> produce a full barrier, the LOCK can be followed by an
> smp_mb__after_unlock_lock() invocation. This will produce a
> full barrier if either (a) the UNLOCK and the LOCK are executed
> by the same CPU or task, or (b) the UNLOCK and LOCK act on the
> same lock variable.

So you're still requiring smp_mb__after_unlock_lock() even if they're on
the same variable?

> The smp_mb__after_unlock_lock() primitive is
> free on many architectures. Without smp_mb__after_unlock_lock(),
> the UNLOCK and LOCK can cross:

Contradicted below :-)

> *A = a;
> UNLOCK M
> LOCK N
> *B = b;
>
> could occur as:
>
> LOCK N, STORE *B, STORE *A, UNLOCK M
>
> With smp_mb__after_unlock_lock(), they cannot, so that:
>
> *A = a;
> UNLOCK M
> LOCK N
> smp_mb__after_unlock_lock();
> *B = b;
>
> will always occur as either of the following:
>
> STORE *A, UNLOCK, LOCK, STORE *B
> STORE *A, LOCK, UNLOCK, STORE *B

See, UNLOCK and LOCK can still cross :-)

> If the UNLOCK and LOCK were instead both operating on the same
> lock variable, only the first of these two alternatives can occur.

Agreed.

Sorry for being a pedant. :-)


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