Messages in this thread |  | | Date | Thu, 16 Nov 2017 11:00:58 +0100 | From | Andrea Parri <> | Subject | Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t |
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On Thu, Nov 16, 2017 at 09:58:04AM +0100, Peter Zijlstra wrote: > On Wed, Nov 15, 2017 at 10:01:11PM +0100, Andrea Parri wrote: > > > > And in specific things like: > > > > > > 135e8c9250dd5 > > > ecf7d01c229d1 > > > > > > which use the release of rq->lock paired with the next acquire of the > > > same rq->lock to match with an smp_rmb(). > > > > Those cycles are currently forbidden by LKMM _when_ you consider the > > smp_mb__after_spinlock() from schedule(). See rfi-rel-acq-is-not-mb > > from my previous email and Alan's remarks about cumul-fence. > > I'm not sure I get your point; and you all seem to forget I do not in > fact speak the ordering lingo. So I have no idea what > rfi-blah-blah or cumul-fence mean.
I expand on my comment. Consider the following test:
C T1
{}
P0(int *x, int *y, spinlock_t *s) { spin_lock(s); WRITE_ONCE(*x, 1); spin_unlock(s); spin_lock(s); WRITE_ONCE(*y, 1); spin_unlock(s); }
P1(int *x, int *y) { int r0; int r1;
r0 = READ_ONCE(*y); smp_rmb(); r1 = READ_ONCE(*x); }
exists (1:r0=1 /\ 1:r1=0)
According to LKMM, the store to x happens before the store to y but there is no guarantee that the former store propagate (to P1) before the latter (which is what we need to forbid that state). As a result, that state in the "exists" clause is _allowed_ by LKMM.
The LKMM encodes happens-before (or execution) ordering with a relation named "hb", while it encodes "propagation ordering" with "cumul-fence".
Andrea
> > I know rel-acq isn't smp_mb() and I don't think any of the above patches > need it to be. They just need it do be a local ordering, no? > > Even without smp_mb__after_spinlock() we get that: > > spin_lock(&x) > x = 1 > spin_unlock(&x) > spin_lock(&x) > y = 1 > spin_unlock(&x) > > guarantees that x happens-before y, right? > > And that should be sufficient to then order something else against, like > for example: > > r2 = y > smp_rmb() > r1 = x > > no? > >
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