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