Messages in this thread | | | Date | Wed, 29 Apr 2015 15:58:36 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v3] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write |
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On 04/28/2015 02:17 PM, Davidlohr Bueso wrote: > On Mon, 2015-04-27 at 16:25 -0400, Waiman Long wrote: >>>> + /* >>>> + * up_write() cleared the owner field before calling this function. >>>> + * If that field is now set, a writer must have stolen the lock and >>>> + * the wakeup operation should be aborted. >>>> + */ >>>> + if (rwsem_has_active_writer(sem)) >>>> + goto out; >>> We currently allow small races between rwsem owner and counter checks. >>> And __rwsem_do_wake() can be called by checking the former -- and lock >>> stealing is done with the counter as well. Please see below how we back >>> out of such cases, as it is very much considered when granting the next >>> reader. So nack to this as is, sorry. >> If the first one in the queue is a writer, wake_up_process() may be >> called directly which can be quite expensive if the lock has already >> been stolen as the task will have to sleep again. > But how can this occur? Lock stealing takes form in two places: > > 1) fastpath: only if the counter is 0 -- which, since we are discussing > waking up waiter(s) code, obviously cannot occur. > > 2) With the cmpxchg() in rwsem_try_write_lock(), which is serialized > with the wait_lock, so again this cannot occur. > > Which is why this is not considered in __rwsem_do_wake() when waking the > writer fist in the queue. > > Thanks, > Davidlohr >
A write lock can also be acquired by a spinning writer in rwsem_try_write_lock_unqueued() where wait_lock isn't used. With multiple down_read's, it is possible that the first exiting reader wakes up a writer who acquires the write lock while the other readers are waiting for acquiring the wait_lock.
Cheers, Longman
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