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Date: Fri, 20 May 2016 17:59:21 -0700 From: Davidlohr Bueso <dave@...olabs.net> To: Peter Zijlstra <peterz@...radead.org> Cc: Waiman Long <waiman.long@....com>, manfred@...orfullife.com, mingo@...nel.org, torvalds@...ux-foundation.org, ggherdovich@...e.com, mgorman@...hsingularity.net, linux-kernel@...r.kernel.org Subject: Re: sem_lock() vs qspinlocks 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 of when the lock is and isn't contended (as returned to by the user). Ie avoiding crap like: atomic_read(&lock->val) && atomic_read(&lock->val) != _Q_LOCKED_VAL In any case, falsely returning for the 'locked, uncontended' case, vs completely ignoring waiters is probably the lesser evil :). Thanks, Davidlohr
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