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

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Date:   Thu, 21 Sep 2017 12:50:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 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.

Thanks!

-- Steve