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SubjectRE: [PATCH] Profile likely/unlikely macros
Date


> Andrew Morton wrote:
> > Daniel Walker <dwalker@mvista.com> wrote:
> >
> >> + if (likeliness->type & LIKELY_UNSEEN) {
> >> + if (atomic_dec_and_test(&likely_lock)) {
> >> + if (likeliness->type & LIKELY_UNSEEN) {
> >> + likeliness->type &= (~LIKELY_UNSEEN);
> >> + likeliness->next = likeliness_head;
> >> + likeliness_head = likeliness;
> >> + }
> >> + }
> >> + atomic_inc(&likely_lock);
> >
> >
> > hm, good enough I guess. It does need a comment explaining why we
> > don't just do spin_lock().
>
> I guess it is so it can be used in NMIs and interrupts
> without turning interrupts off (so is somewhat lightweight).
>
> But please Daniel, just use spinlocks and trylock. This is
> buggy because it doesn't get the required release consistency correct.

Could you elaborate a bit what's wrong here? (memory barriers, etc? What about the test_and_set_bit() thing Andrew suggested?)

Trylock is a bit more dirty because we need to avoid recursion (it used likely/unlikely too). While there are ways to work around
it, atomic operations seem to be cleaner.

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