linux-kernel - Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

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Message-ID: <20160929144937.GA5016@twins.programming.kicks-ass.net>
Date:   Thu, 29 Sep 2016 16:49:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>, mingo@...nel.org,
        juri.lelli@....com, xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH -v2 1/9] rtmutex: Deboost before waking up the top waiter

On Thu, Sep 29, 2016 at 10:43:54AM -0400, Thomas Gleixner wrote:
> On Mon, 26 Sep 2016, Peter Zijlstra wrote:
> 
> > On Mon, Sep 26, 2016 at 11:37:27AM -0400, Steven Rostedt wrote:
> > > On Mon, 26 Sep 2016 11:35:03 -0400
> > > Steven Rostedt <rostedt@...dmis.org> wrote:
> > > 
> > > > Especially now that the code after the spin_unlock(&hb->lock) is now a
> > > > critical section (preemption is disable). There's nothing obvious in
> > > > futex.c that says it is.
> > > 
> > > Not to mention, this looks like it will break PREEMPT_RT as wake_up_q()
> > > calls sleepable spin locks.
> > 
> > What locks would that be?
> 
> None :)
> 
> It still breaks RT in the futex case due to:
> 
>    deboost = rt_mutex_futex_unlock();
> 
>    spin_unlock(&hb->lock);
> 	....

Yeah, noticed that already. Am currently trying to untangle the pi_state
locking rules.