linux-kernel - Re: [PATCH RT] locking/rtmutex: don't drop the wait

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Message-ID: <20170921173512.sd6scop62epcmid5@linutronix.de>
Date:   Thu, 21 Sep 2017 19:35:12 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de
Subject: Re: [PATCH RT] locking/rtmutex: don't drop the wait_lock twice

On 2017-09-21 12:50:27 [-0400], Steven Rostedt wrote:
> On Thu, 21 Sep 2017 18:43:02 +0200
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > On 2017-09-21 12:31:05 [-0400], Steven Rostedt wrote:
> > > > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > > > index f03876322d4a..79f49d73e4d0 100644
> > > > --- a/kernel/locking/rtmutex.c
> > > > +++ b/kernel/locking/rtmutex.c
> > > > @@ -2281,7 +2281,6 @@ int __rt_mutex_start_proxy_lock(struct rt_mutex *lock,
> > > >  	raw_spin_lock(&task->pi_lock);
> > > >  	if (task->pi_blocked_on) {
> > > >  		raw_spin_unlock(&task->pi_lock);
> > > > -		raw_spin_unlock_irq(&lock->wait_lock);  
> > > 
> > > Hmm, before this patch, irqs are enabled when returning with -EAGAIN.
> > > But now they are not. Should that be:
> > > 
> > > 		raw_spin_unlock_irq(&taks->pi_lock);
> > > 
> > > or is there something that changes this?  
> > 
> > There is something else. Before that futex rework there was just
> > rt_mutex_start_proxy_lock() and it did lock & unlock of ->wait_lock.
> > This no longer the case after the rework. So now the caller does this.
> >
> 
> So this actually fixes two bugs then? Anyway, probably want to add that
> in the change log to explain why it is ok to change the irq semantics
> here too.

No, it does not. It fixes only one and this one was introduced while I
rebased RT ontop of the futex work - the patch "futex: Fix bug on when a
requeued RT task times out" to be exact.
If you look at the code in the v4.9 or v4.11 RT then you see that there
is just rt_mutex_start_proxy_lock()() and this function acquires and
releases ->wait_lock lock. After the futex rework the locking changed
and I missed to adapt the RT-only patch I mentioned.
Again: without the patch, the ->wait_lock is dropped twice in the error
case here: once here and the second time by the caller and this has only
been like this since the futex-rework. So this does not apply to v4.1-RT
for instance because the futex rework got into v4.9.18-rt14 and I don't
recall that you backported it.

> Thanks!
> 
> -- Steve

Sebastian