linux-kernel - Re: [PATCH v4 3/5] locking/rwsem: Don't wake up one's own task

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Message-ID: <20160518103011.GC3193@twins.programming.kicks-ass.net>
Date:	Wed, 18 May 2016 12:30:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v4 3/5] locking/rwsem: Don't wake up one's own task

On Tue, May 17, 2016 at 09:26:21PM -0400, Waiman Long wrote:
> As rwsem_down_read_failed() will queue itself and potentially call
> __rwsem_do_wake(sem, RWSEM_WAKE_ANY), it is possible that a reader
> will try to wake up its own task. This patch adds a check to make
> sure that this won't happen.
> 

Yes, this is 'weird', but why are we fixing it at the cost of an extra
branch?