Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered

[Thomas Gleixner]

On Mon, 28 Jan 2019, Peter Zijlstra wrote:
> On Mon, Jan 28, 2019 at 02:44:10PM +0100, Peter Zijlstra wrote:
> > On Thu, Nov 29, 2018 at 12:23:21PM +0100, Heiko Carstens wrote:
> > 
> > > And indeed, if I run only this test case in an endless loop and do
> > > some parallel work (like kernel compile) it currently seems to be
> > > possible to reproduce the warning:
> > > 
> > > while true; do time ./testrun.sh nptl/tst-robustpi8 --direct ; done
> > > 
> > > within the build directory of glibc (2.28).
> > 
> > Right; so that reproduces for me.
> > 
> > After staring at all that for a while; trying to remember how it all
> > worked (or supposed to work rather), I became suspiscous of commit:
> > 
> >   56222b212e8e ("futex: Drop hb->lock before enqueueing on the rtmutex")
> > 
> > And indeed, when I revert that; the above reproducer no longer works (as
> > in, it no longer triggers in minutes and has -- so far -- held up for an
> > hour+ or so).

Right after staring long enough at it, the commit simply forgot to give
__rt_mutex_start_proxy_lock() the same treatment as it gave to
rt_mutex_wait_proxy_lock().

Patch below cures that.

Thanks,

        tglx

8<----------------

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2845,7 +2845,7 @@ static int futex_lock_pi(u32 __user *uad
                ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
                /* Fixup the trylock return value: */
                ret = ret ? 0 : -EWOULDBLOCK;
-               goto no_block;
+               goto cleanup;
        }
 
        rt_mutex_init_waiter(&rt_waiter);
@@ -2870,17 +2870,15 @@ static int futex_lock_pi(u32 __user *uad
        if (ret) {
                if (ret == 1)
                        ret = 0;
-
-               spin_lock(q.lock_ptr);
                goto no_block;
        }
 
-
        if (unlikely(to))
                hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS);
 
        ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
 
+no_block:
        spin_lock(q.lock_ptr);
        /*
         * If we failed to acquire the lock (signal/timeout), we must
@@ -2894,7 +2892,7 @@ static int futex_lock_pi(u32 __user *uad
        if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, 
&rt_waiter))
                ret = 0;
 
-no_block:
+cleanup:
        /*
         * Fixup the pi_state owner and possibly acquire the lock if we
         * haven't already.
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1749,9 +1749,6 @@ int __rt_mutex_start_proxy_lock(struct r
                ret = 0;
        }
 
-       if (unlikely(ret))
-               remove_waiter(lock, waiter);
-
        debug_rt_mutex_print_deadlock(waiter);
 
        return ret;
@@ -1778,6 +1775,8 @@ int rt_mutex_start_proxy_lock(struct rt_
 
        raw_spin_lock_irq(&lock->wait_lock);
        ret = __rt_mutex_start_proxy_lock(lock, waiter, task);
+       if (unlikely(ret))
+               remove_waiter(lock, waiter);
        raw_spin_unlock_irq(&lock->wait_lock);
 
        return ret;