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