Messages in this thread | | | Date | Sat, 21 May 2016 09:40:21 +0200 | From | Peter Zijlstra <> | Subject | Re: sem_lock() vs qspinlocks |
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On Sat, May 21, 2016 at 12:01:00AM -0400, Waiman Long wrote: > On 05/20/2016 08:59 PM, Davidlohr Bueso wrote: > >On Fri, 20 May 2016, Peter Zijlstra wrote: > > > >>On Fri, May 20, 2016 at 04:47:43PM -0400, Waiman Long wrote: > >> > >>>>Similarly, and I know you hate it, but afaict, then semantically > >>>>queued_spin_is_contended() ought to be: > >>>> > >>>>- return atomic_read(&lock->val) & ~_Q_LOCKED_MASK; > >>>>+ return atomic_read(&lock->val); > >>>> > >> > >>>Looking for contended lock, you need to consider the lock waiters > >>>also. So > >>>looking at the whole word is right. > >> > >>No, you _only_ need to look at the lock waiters. > > > >Is there anyway to do this in a single atomic_read? My thought is that > >otherwise > >we could further expand the race window
Its inherently racy, arrival of a contender is subject to timing. No point in trying to fix what can't be fixed.
> The existing code is doing that, but I would argue that including the > locked, but uncontended case isn't a bad idea.
It _IS_ a bad idea, you get unconditional lock-breaks.
Its the same as:
#define spin_is_contended(l) (true)
Because the only reason you're using spin_is_conteded() is if you're holding it.
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