On Mon, Jul 31, 2017 at 11:05:35AM +0200, Peter Zijlstra wrote: > On Wed, Jul 26, 2017 at 08:47:50PM +0800, Boqun Feng wrote: > > > > + > > > +Further, while something like: > > > + > > > + smp_mb__before_atomic(); > > > + atomic_dec(&X); > > > + > > > +is a 'typical' RELEASE pattern, the barrier is strictly stronger than > > > +a RELEASE. Similarly for something like: > > > + > > > > .. at here. Maybe you planned to put stronger ACQUIRE pattern? > > Yes, although I struggled to find a sensible one. The problem is that > ACQUIRE is on loads and value returning atomics have an ACQUIRE variant, > so why would you ever want to use smp_mb__after_atomic() for this. > > > That is, the best I could come up with is something like: > > val = atomic_fetch_or_relaxed(1, &var); > smp_mb__after_atomic(); > > But in that case we should've just written: > > val = atomic_fetch_or_acquire(1, &var); >
Agreed.
And besides, in memory-barriers.txt, the wording is:
(*) smp_mb__before_atomic();
(*) smp_mb__after_atomic();
These are for use with atomic (such as add, subtract, increment and
decrement) functions that don't return a value, especially when used for
reference counting.
So actually, using smp_mb__after_atomic() for ACQUIRE is a misuse.
>
> Suggestions?
As a result, I think it's better we say smp_mb__{before,after}_atomic()
are only for 1) non-value-returning RmW atomic ops, 2)
{set,clear,change}_bit and 3) internal use of atomic primitives(e.g. the
generic version of fully ordered atomics).
1) prevents people to use it for an ACQUIRE, but allows for a RELEASE.
1) & 2) makes atomic_t.txt consistent with memory-barriers.txt
3) explains our usage of those barriers internally.
Thoughts?
Regards,
Boqun
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