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

[Waiman Long]

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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