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SubjectRe: [PATCH RFC LKMM 1/7] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
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

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