linux-kernel - Re: [PATCH RT] kernel/futex: don't deboost too early

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Message-ID: <20161002060137.2716b089@grimm.local.home>
Date:   Sun, 2 Oct 2016 06:01:37 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org
Subject: Re: [PATCH RT] kernel/futex: don't deboost too early

On Fri, 30 Sep 2016 20:43:26 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Fri, 30 Sep 2016, Steven Rostedt wrote:
> > This looks awfully complex. Would something as simple as this work?
> > 
> > What harm can happen by moving the holding of the lock after the
> > wakeups for RT?  
> 
> That's exactly bringing us back to the state before we added the delayed
> wakeup so that the woken waiter will not be blocked on hb->lock right
> away. That's 2 extra context switches for nothing.

Ah crap, that's the wake up of the owner that's about to grab the lock,
in which case (if on the same CPU) may preempt this guy, just to grab
the hb->lock and block again.

Grumble, how come the easy way is never a possibility :-p

-- Steve