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Message-ID: <1398695832.14475.10.camel@marge.simpson.net>
Subject: Re: [ANNOUNCE] 3.14-rt1
From: Mike Galbraith <umgwanakikbuti@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Nicholas Mc Guire <der.herr@hofr.at>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, John Kacur <jkacur@redhat.com>
Date: Mon, 28 Apr 2014 16:37:12 +0200
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On Mon, 2014-04-28 at 10:18 -0400, Steven Rostedt wrote: 
> On Mon, 28 Apr 2014 11:09:46 +0200
> Mike Galbraith <umgwanakikbuti@gmail.com> wrote:
>  
> > migrate_disable-pushd-down-in-atomic_dec_and_spin_lo.patch
> > 
> > bug: migrate_disable() after blocking is too late.
> > 
> > @@ -1028,12 +1028,12 @@ int atomic_dec_and_spin_lock(atomic_t *a
> >         /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
> >         if (atomic_add_unless(atomic, -1, 1))
> >                 return 0;
> > -       migrate_disable();
> >         rt_spin_lock(lock);
> > -       if (atomic_dec_and_test(atomic))
> > +       if (atomic_dec_and_test(atomic)){
> > +               migrate_disable();
> 
> Makes sense, as the CPU can go offline right after the lock is grabbed
> and before the migrate_disable() is called.
> 
> Seems that migrate_disable() must be called before taking the lock as
> it is done in every other location.

And for tasklist_lock, seems you also MUST do that prior to trylock as
well, else you'll run afoul of the hotplug beast.

-Mike