Messages in this thread | | | Date | Tue, 18 Feb 2014 19:58:49 -0500 | From | Waiman Long <> | Subject | Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation |
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On 02/18/2014 04:37 PM, Peter Zijlstra wrote: > On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote: >>>> + /* >>>> + * At the head of the wait queue now >>>> + */ >>>> + while (true) { >>>> + u32 qcode; >>>> + int retval; >>>> + >>>> + retval = queue_get_lock_qcode(lock,&qcode, my_qcode); >>>> + if (retval> 0) >>>> + ; /* Lock not available yet */ >>>> + else if (retval< 0) >>>> + /* Lock taken, can release the node& return */ >>>> + goto release_node; >>>> + else if (qcode != my_qcode) { >>>> + /* >>>> + * Just get the lock with other spinners waiting >>>> + * in the queue. >>>> + */ >>>> + if (queue_spin_trylock_unfair(lock)) >>>> + goto notify_next; >>> Why is this an option at all? >>> >>> >> Are you referring to the case (qcode != my_qcode)? This condition will be >> true if more than one tasks have queued up. > But in no case should we revert to unfair spinning or stealing. We > should always respect the queueing order. > > If the lock tail no longer points to us, then there's further waiters > and we should wait for ->next and unlock it -- after we've taken the > lock. >
A task will be in this loop when it is already the head of a queue and is entitled to take the lock. The condition (qcode != my_qcode) is to decide whether it should just take the lock or take the lock & clear the code simultaneously. I am a bit cautious to use queue_spin_trylock_unfair() as there is a possibility that a CPU may run out of the queue node and need to do unfair busy spinning.
-Longman
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