linux-kernel - Re: [PATCH v3] locking/rwsem: reduce spinlock contention in wakeup after up_read/up

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Message-ID: <1430244355.2004.14.camel@stgolabs.net>
Date:	Tue, 28 Apr 2015 11:05:55 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v3] locking/rwsem: reduce spinlock contention in wakeup
 after up_read/up_write

On Tue, 2015-04-28 at 10:50 -0700, Jason Low wrote:
> On Tue, 2015-04-28 at 19:17 +0200, Peter Zijlstra wrote:
> 
> > To me it makes more sense to reverse these two branches (identical code
> > wise of course) and put the special case first.
> > 
> > Alternatively we could also do something like the below, which to my
> > eyes looks a little better still, but I don't care too much.
> > 
> > 	if (rwsem_has_spinner(sem)) {
> > 		/*
> > 		 * comment ...
> > 		 */
> > 		 smp_rmb();
> > 		 if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
> > 			return sem;
> > 		 goto locked;
> > 	}
> > 
> > 	raw_spin_lock_irqsave(&sem->wait_lock, flags);
> > locked:
> 
> How about putting this into its own function:

I'd actually prefer reusing the current function. We have so many
utility functions that we've over-encapsulated the code a bit, it makes
reading _harder_, imho.

Thanks,
Davidlohr

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