Messages in this thread | | | Date | Thu, 23 Apr 2015 14:41:29 -0400 | From | Waiman Long <> | Subject | Re: [PATCH] locking/rwsem: reduce spinlock contention in wakeup after up_read/up_write |
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On 04/18/2015 11:40 AM, Peter Zijlstra wrote: > 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.
I am sorry that I am a bit sloppy here. I have just sent out an updated patch to remedy this. I have added a smp_mb__after_atomic() to ensure proper memory ordering. However, I am not so sure if this primitive or just a simple smp_rmb() will be more expensive in other non-x86 architectures.
Cheers, Longman
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