Messages in this thread | | | Date | Tue, 10 Apr 2018 11:35:51 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath |
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On Mon, Apr 09, 2018 at 06:19:59PM +0100, Will Deacon wrote: > On Mon, Apr 09, 2018 at 05:54:20PM +0200, Peter Zijlstra wrote: > > On Mon, Apr 09, 2018 at 03:54:09PM +0100, Will Deacon wrote: > > > @@ -289,18 +315,26 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) > > > return; > > > > > > /* > > > - * If we observe any contention; queue. > > > + * If we observe queueing, then queue ourselves. > > > */ > > > - if (val & ~_Q_LOCKED_MASK) > > > + if (val & _Q_TAIL_MASK) > > > goto queue; > > > > > > /* > > > + * We didn't see any queueing, so have one more try at snatching > > > + * the lock in case it became available whilst we were taking the > > > + * slow path. > > > + */ > > > + if (queued_spin_trylock(lock)) > > > + return; > > > + > > > + /* > > > * trylock || pending > > > * > > > * 0,0,0 -> 0,0,1 ; trylock > > > * 0,0,1 -> 0,1,1 ; pending > > > */ > > > + val = set_pending_fetch_acquire(lock); > > > if (!(val & ~_Q_LOCKED_MASK)) { > > > > So, if I remember that partial paper correctly, the atomc_read_acquire() > > can see 'arbitrary' old values for everything except the pending byte, > > which it just wrote and will fwd into our load, right? > > > > But I think coherence requires the read to not be older than the one > > observed by the trylock before (since it uses c-cas its acquire can be > > elided). > > > > I think this means we can miss a concurrent unlock vs the fetch_or. And > > I think that's fine, if we still see the lock set we'll needlessly 'wait' > > for it go become unlocked. > > Ah, but there is a related case that doesn't work. If the lock becomes > free just before we set pending, then another CPU can succeed on the > fastpath. We'll then set pending, but the lockword we get back may still > have the locked byte of 0, so two people end up holding the lock. > > I think it's worth giving this a go with the added trylock, but I can't > see a way to avoid the atomic_fetch_or at the moment.
Oh yikes, indeed. Yeah, I don't see how we'd be able to fix that one.
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