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