Messages in this thread | | | Date | Mon, 15 Feb 2016 22:30:34 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin() |
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On 02/15/2016 10:00 PM, Jason Low wrote: > On Mon, 2016-02-15 at 18:55 -0500, Waiman Long wrote: >> On 02/12/2016 03:40 PM, Peter Zijlstra wrote: >>> On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote: >>>> @@ -358,8 +373,8 @@ static bool mutex_optimistic_spin(struct mutex *lock, >>>> } >>>> >>>> mutex_set_owner(lock); >>>> - osq_unlock(&lock->osq); >>>> - return true; >>>> + acquired = true; >>>> + break; >>>> } >>>> >>>> /* >>>> @@ -380,7 +395,10 @@ static bool mutex_optimistic_spin(struct mutex *lock, >>>> cpu_relax_lowlatency(); >>>> } >>>> >>>> - osq_unlock(&lock->osq); >>>> + if (!waiter) >>>> + osq_unlock(&lock->osq); >>>> + if (acquired || waiter) >>>> + return acquired; >>>> done: >>>> /* >>>> * If we fell out of the spin path because of need_resched(), >>> Is there a reason to not also preempt in the wait-loop? Surely the same >>> reason is still valid there too? >> The waiter does check for need_sched(). So it will break out of the loop >> and return false in this case. This causes the waiter to loop back and >> goes to sleep if the lock can't be acquired. That is why I don't think >> we need to do another schedule_preempt_disabled() here. > The purpose of the additional reschedule point is to avoid delaying > preemption, which still applies if the spinner is a waiter. If it is a > waiter, the difference is that the delay isn't as long since it doesn't > need to be added to the wait_list. Nonetheless, preemption delays can > still occur, so I think the additional preemption point should also be > there in the waiter case.
You are right. Taking the wait lock can introduce arbitrary delay. So I will modify the patch to fall through and check for preemption.
Cheers, Longman
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