Messages in this thread | | | Date | Sat, 18 Apr 2015 17:40:14 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write |
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On Fri, Apr 17, 2015 at 10:03:18PM -0400, Waiman Long wrote: > @@ -478,7 +515,28 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) > { > unsigned long flags; > > - raw_spin_lock_irqsave(&sem->wait_lock, flags); > + /* > + * If a spinner is present, it is not necessary to do the wakeup. > + * Try to do wakeup only if the trylock succeeds to minimize > + * spinlock contention which may introduce too much delay in the > + * unlock operation. > + * > + * In case the spinning writer is just going to break out of the > + * waiting loop, it will still do a trylock in > + * rwsem_down_write_failed() before sleeping. > + * IOW, if rwsem_has_spinner() is true, it will guarantee at least > + * one trylock attempt on the rwsem.
successful trylock? I think we're having 'issues' on if failed trylocks (and cmpxchg) imply full barriers.
> + * > + * spinning writer > + * --------------- > + * [S] osq_unlock() > + * MB > + * [RmW] rwsem_try_write_lock() > + */
Ordering comes in pairs, this is incomplete.
> + if (!rwsem_has_spinner(sem)) > + raw_spin_lock_irqsave(&sem->wait_lock, flags); > + else if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags)) > + return sem; > > /* do nothing if list empty */ > if (!list_empty(&sem->wait_list)) > -- > 1.7.1 >
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