Messages in this thread | | | Subject | Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() | From | Jason Low <> | Date | Mon, 15 Feb 2016 18:22:14 -0800 |
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On Mon, 2016-02-15 at 18:15 -0800, Jason Low wrote: > On Fri, 2016-02-12 at 14:14 -0800, Davidlohr Bueso wrote: > > On Fri, 12 Feb 2016, Peter Zijlstra wrote: > > > > >On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote: > > >> static bool mutex_optimistic_spin(struct mutex *lock, > > >> + struct ww_acquire_ctx *ww_ctx, > > >> + const bool use_ww_ctx, int waiter) > > >> { > > >> struct task_struct *task = current; > > >> + bool acquired = false; > > >> > > >> + if (!waiter) { > > >> + if (!mutex_can_spin_on_owner(lock)) > > >> + goto done; > > > > > >Why doesn't the waiter have to check mutex_can_spin_on_owner() ? > > > > afaict because mutex_can_spin_on_owner() fails immediately when the counter > > is -1, which is a nono for the waiters case. > > mutex_can_spin_on_owner() returns false if the task needs to reschedule > or if the lock owner is not on_cpu. In either case, the task will end up > not spinning when it enters the spin loop. So it makes sense if the > waiter also checks mutex_can_spin_on_owner() so that the optimistic spin > queue overhead can be avoided in those cases.
Actually, since waiters bypass the optimistic spin queue, that means the the mutex_can_spin_on_owner() isn't really beneficial. So Waiman is right in that it's fine to skip this in the waiter case.
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