linux-kernel - Re: [PATCH] rtmutex: ensure only the top waiter or higher priority task can take the lock and reduce unrelated boosting

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Date:	Thu, 16 Dec 2010 09:07:49 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <hidave.darkstar@...il.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Namhyung Kim <namhyung@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] rtmutex: ensure only the top waiter or higher priority
 task can take the lock and reduce unrelated boosting

On Thu, 2010-12-16 at 09:14 +0800, Lai Jiangshan wrote:

> It has called try_to_take_rt_mutex() in __rt_mutex_slowlock(),
> when timeout or got signal, it returns from __rt_mutex_slowlock()
> with lock->wait_lock still held, and then calls remove_waiter(),
> 
> so we don't need to call try_to_take_rt_mutex() in remove_waiter().
> It is strange that remove_waiter() do some "require lock" work.

I think you are correct here. It should never get to this path where
!owner && first is true. If we timed out then we either got the lock or
we could not. If we could not than owner had to be set or we are not the
top waiter. Thus we need:

	if (!owner) {
		BUG_ON(first);
		return;
	}

-- Steve


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