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

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Date:	Wed, 18 May 2016 12:04:06 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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 05/18/2016 06:30 AM, Peter Zijlstra wrote:
> 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?

I think I should have put an unlikely tag there. Anyway, the cost of an 
extra branch should be pretty small compared with all the task wakeup 
work that have to be done. If you think this is necessary, I am totally 
fine for scrapping it.

Cheers,
Longman