Re: [PATCH RFC] rtmutex: Permit rt_mutex_unlock() to be invoked with irqs disabled

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Aug 19, 2011 at 09:31:05PM -0400, Arnaud Lacombe wrote:
> Hi,
> 
> On Sun, Jul 24, 2011 at 11:56 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Sun, Jul 24, 2011 at 11:00:41AM +0200, Thomas Gleixner wrote:
> >> On Sat, 23 Jul 2011, Paul E. McKenney wrote:
> >> > On Sun, Jul 24, 2011 at 02:05:13AM +0200, Thomas Gleixner wrote:
> >> > > On Sun, 24 Jul 2011, Thomas Gleixner wrote:
> >> > > > > > Thomas, I'm inclined to merge this, any objections?
> >> > > > >
> >> > > > > FWIW, it has been passing tests here.
> >> > > >
> >> > > > If it's only the unlock path, I'm fine with that change.
> >> > > >
> >> > > > Acked-by-me
> >> > >
> >> > > Hrmpft. That's requiring all places to take the lock irq safe. Not
> >> > > really amused. For -RT that's a hotpath and we can really do without
> >> > > the irq fiddling there. That needs a bit more thought.
> >> >
> >> > Indeed...  If I make only some of the lock acquisitions irq safe, lockdep
> >> > will yell at me.  And rightfully so, as that could result in deadlock.
> >> >
> >> > So, what did you have in mind?
> >>
> >> Have no real good idea yet for this. Could you grab rt and check
> >> whether you can observe any impact when the patch is applied?
> >
> > Hmmm, wait a minute...  There might be a way to do this with zero
> > impact on the fastpath, given that I am allocating an rt_mutex on
> > the stack that is used only by RCU priority boosting, and that only
> > rt_mutex_init_proxy_locked(), rt_mutex_lock(), and rt_mutex_unlock()
> > are used.
> >
> > So I could do the following:
> >
> > o       Use lockdep_set_class_and_name() to make the ->wait_lock()
> >        field of my rt_mutex have a separate lockdep class.  I guess
> >        I should allocate a global variable for lock_class_key
> >        rather than allocating it on the stack.  ;-)
> >
> > o       Make all calls from RCU priority boosting to rt_mutex_lock()
> >        and rt_mutex_unlock() have irqs disabled.
> >
> > o       Make __rt_mutex_slowlock() do the following when sleeping:
> >
> >        raw_spin_unlock(&lock->wait_lock);
> >
> >        debug_rt_mutex_print_deadlock(waiter);
> >
> >        {
> >                int was_disabled = irqs_disabled();
> >
> >                if (was_disabled)
> >                        local_irq_enable();
> >
> FWIW, the final construct you opted for in -next:
> 
>     if (was_disabled = irqs_disabled())
>         local_irq_enable();
> 
> triggers:
> 
> /linux/linux/kernel/rtmutex.c: In function '__rt_mutex_slowlock':
> /linux/linux/kernel/rtmutex.c:605:3: warning: suggest parentheses
> around assignment used as truth value

But I -do- have parentheses around that assignment!!!

Sigh, gcc strikes again.  Does the following patch help?  If so, I will
fold it into commit 83841f02.

							Thanx, Paul

------------------------------------------------------------------------

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index 0222e34..2548f44 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -602,7 +602,8 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
 
 		raw_spin_unlock(&lock->wait_lock);
 
-		if (was_disabled = irqs_disabled())
+		was_disabled = irqs_disabled();
+		if (was_disabled)
 			local_irq_enable();
 
 		debug_rt_mutex_print_deadlock(waiter);