Messages in this thread | | | Date | Mon, 2 Nov 2015 13:11:22 -0800 | Subject | Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() | From | Linus Torvalds <> |
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On Mon, Nov 2, 2015 at 12:36 PM, David Howells <dhowells@redhat.com> wrote: > Linus Torvalds <torvalds@linux-foundation.org> wrote: > >> > + smp_read_barrier_depends(); /* ctrl */ \ >> > + smp_rmb(); /* ctrl + rmb := acquire */ \ > > Doesn't smp_rmb() imply an smp_read_barrier_depends() anyway?
Yes, it does. But that "smp_read_barrier_depends()" is actually mis-used as a "barrier against subsequent dependent writes, thanks to the control flow". It's not protecting against subsequent reads - which is what the smp_rmb() is about.
Which is completely bogus, but that's what the comment implies.
Of course, on alpha (which is where smp_read_barrier_depends() makes a difference), both that and smp_rmb() are just full memory barriers, because alpha is some crazy sh*t. So yes, a "smp_rmb()" is sufficient everywhere, but that is actually not where the confusion comes from in the first place.
Linus
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