linux-kernel - Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

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Date:	Wed, 4 Feb 2015 13:06:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Jason Low <jason.low2@...com>, Davidlohr Bueso <dave@...olabs.net>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

On Tue, Feb 03, 2015 at 11:43:36AM -0800, Tim Chen wrote:
> That's true.  We cannot have the lock grabbed by a new write 
> contender as any new writer contender of the lock will be 
> queued by the OSQ logic. Only the
> thread doing the optimistic spin is attempting write lock.  
> In other word, switching of write owner of the rwsem to a new
> owner cannot happen.  Either write owner stay as the original one, or
> we don't have a write owner.  So using test of write owner
> switching as an indicator of congestion is incorrect.
> 
> If my reasoning above is sound, then the check 
> 
> +       if (READ_ONCE(sem->owner))
> +               return true; /* new owner, continue spinning */
> +
> 
> is unnecessary and can be removed, as we cannot have a 
> new write owner of the rwsem, other than the thread
> doing optimistic spinning.

I have read the rest of the thread; but the one thing that I didn't see
is trylocks, trylocks can always come in an steal things regardless of
the OSQ stuff.
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