On Mon, May 11, 2020 at 10:29:18AM -0700, Paul E. McKenney wrote:
> On Mon, May 11, 2020 at 05:52:17PM +0100, Will Deacon wrote:
> > On Mon, May 11, 2020 at 09:43:19AM -0700, Paul E. McKenney wrote:
> > > On Mon, May 11, 2020 at 04:58:13PM +0100, Will Deacon wrote:
> > > > On Sat, May 09, 2020 at 02:36:54PM -0700, Paul E. McKenney wrote:
> > > > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > > > > index 1f77349..1de006e 100644
> > > > > --- a/kernel/locking/osq_lock.c
> > > > > +++ b/kernel/locking/osq_lock.c
> > > > > @@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> > > > > */
> > > > >
> > > > > for (;;) {
> > > > > - if (prev->next == node &&
> > > > > + /*
> > > > > + * cpu_relax() below implies a compiler barrier which
> > > > > would
> > > > > + * prevent this comparison being optimized away.
> > > > > + */
> > > > > + if (data_race(prev->next) == node &&
> > > > > cmpxchg(&prev->next, node, NULL) == node)
> > > > > break;
> > > >
> > > > I'm fine with the data_race() placement, but I don't find the comment
> > > > very helpful. We assign the result of a READ_ONCE() to 'prev' in the
> > > > loop, so I don't think that the cpu_relax() is really relevant.
> > >
> > > Suppose that the compiler loaded a value that was not equal to "node".
> > > In that case, the cmpxchg() won't happen, so something else must force
> > > the compiler to do the reload in order to avoid an infinite loop, right?
> > > Or am I missing something here?
> >
> > Then we just go round the loop and reload prev:
> >
> > prev = READ_ONCE(node->prev);
> >
> > which should be enough to stop the compiler, no?
>
> Yes, that would also work. Either have the cpu_relax() or a barrier()
> or whatever on the one hand, or, as you say, turn the data_race() into
> a READ_ONCE(). I personally prefer the READ_ONCE() myself, unless that
> would undesirably suppress other KCSAN warnings.
No, I mean here is the code after this patch is applied:
for (;;) {
if (data_race(prev->next) == node &&
cmpxchg(&prev->next, node, NULL) == node)
break;
/*
* We can only fail the cmpxchg() racing against an unlock(),
* in which case we should observe @node->locked becomming
* true.
*/
if (smp_load_acquire(&node->locked))
return true;
cpu_relax();
/*
* Or we race against a concurrent unqueue()'s step-B, in which
* case its step-C will write us a new @node->prev pointer.
*/
prev = READ_ONCE(node->prev);
}
I'm saying that this READ_ONCE at the end of the loop should be sufficient
to stop the compiler making value assumptions about prev->next. Do you
agree?
Will
