linux-kernel - improve futex on -RT by avoiding the double wake-up

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Message-Id: <1428419030-20030-1-git-send-email-bigeasy@linutronix.de>
Date:	Tue,  7 Apr 2015 17:03:47 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <darren@...art.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com,
	Davidlohr Bueso <davidlohr@...com>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: improve futex on -RT by avoiding the double wake-up

On -RT we see a double wake up if the waiter has a higher priority than
the process doing the wake up. Patch #1 avoids in the PI wake code, #2 in
the non-PI variant (and only for one waiter).

#3 is a small optimisation for message queues avoid a "cpu_relax()" loop.

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