On Tue, Dec 10, 2013 at 06:43:45PM +0100, Peter Zijlstra wrote:
> 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?
Yep!
> > 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 :-)
Good eyes! I changed this to:
The smp_mb__after_unlock_lock() primitive is free on many
architectures. Without smp_mb__after_unlock_lock(), the critical
sections corresponding to the UNLOCK and the LOCK can cross:
Is that better?
> > *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 :-)
Indeed they can! ;-)
> > 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. :-)
;-) ;-) ;-)
Thanx, Paul
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