linux-kernel - Re: [PATCH 4/6] locking/rwsem: Avoid deceiving lock spinners

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Message-ID: <1422573326.2005.7.camel@stgolabs.net>
Date:	Thu, 29 Jan 2015 15:15:26 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] locking/rwsem: Avoid deceiving lock spinners

On Thu, 2015-01-29 at 12:18 -0800, Jason Low wrote:
>  	/*
> -	 * We break out the loop above on need_resched() and when the
> -	 * owner changed, which is a sign for heavy contention. Return
> -	 * success only when lock->owner is NULL.
> +	 * We break out the loop above on either need_resched(), when
> +	 * the owner is not running, or when the lock owner changed.
> +	 * Return success only when the lock owner changed.
>  	 */
> -	return lock->owner == NULL;
> +	return lock->owner != owner;
>  }

Ideally we would refactor all this, along with getting rid of
owner_running() at some point. It no longer makes sense to split up
mutex_spin_on_owner() and we're doing duplicate owner checks. It would
also be simpler than having to guess why we broke out of the loop, for
example.

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