On Fri, Aug 31, 2018 at 10:52:54AM -0400, Alan Stern wrote: > On Fri, 31 Aug 2018, Andrea Parri wrote: > > On Thu, Aug 30, 2018 at 05:31:32PM -0400, Alan Stern wrote: > > > On Thu, 30 Aug 2018, Andrea Parri wrote: > > > > (Remark: ordinary release/acquire are building blocks for code such as > > > > qspinlock, (q)rwlock, mutex, rwsem, ... and what else??). > > > > > > But are these building blocks used the same way for all architectures? > > > > The more, the better! (because then we have the LKMM tools) > > > > We already discussed the "fast path" example: the fast paths of the > > above all resemble: > > > > *_lock(s): atomic_cmpxchg_acquire(&s->val, UNLOCKED_VAL, LOCKED_VAL) ... > > *_unlock(s): ... atomic_set_release(&s->val, UNLOCKED_VAL) > > > > When I read this code, I think "Of course." (unless some arch. has > > messed the implementation of cmpxchg_* up, which can happen...); but > > then I read the subject line of this patch and I think "Wait, what?". > > > > You can argue that this is not generic code, sure; but why on Earth > > would you like to do so?! > > Because the code might not work! On RISC-V, for example, the > implementation of ordinary release/acquire is currently not as strong > as atomic release/acquire. > > Yes, it's true that implementing locks with atomic_cmpxchg_acquire > should be correct on all existing architectures. And Paul has invited > a patch to modify the LKMM accordingly. If you feel that such a change > would be a useful enhancement to the LKMM's applicability, please write > it. Yes, please! That would be the "RmW" discussion which Andrea partially quoted earlier on, so getting that going independently from this patch sounds like a great idea to me. Cheers, Will
