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SubjectRe: [PATCH] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write
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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