futex: Handle transient "ownerless" rtmutex state correctly

Gratian managed to trigger the BUG_ON(!newowner) in fixup_pi_state_owner().
This is one possible chain of events leading to this:

Task Prio       Operation
T1   120	lock(F)
T2   120	lock(F)   -> blocks (top waiter)
T3   50 (RT)	lock(F)   -> boosts T1 and blocks (new top waiter)
XX   		timeout/  -> wakes T2
		signal
T1   50		unlock(F) -> wakes T3 (rtmutex->owner == NULL, waiter bit is set)
T2   120	cleanup   -> try_to_take_mutex() fails because T3 is the top waiter
     			     and the lower priority T2 cannot steal the lock.
     			  -> fixup_pi_state_owner() sees newowner == NULL -> BUG_ON()

The comment states that this is invalid and rt_mutex_real_owner() must
return a non NULL owner when the trylock failed, but in case of a queued
and woken up waiter rt_mutex_real_owner() == NULL is a valid transient
state. The higher priority waiter has simply not yet managed to take over
the rtmutex.

The BUG_ON() is therefore wrong and this is just another retry condition in
fixup_pi_state_owner().

Drop the locks, so that T3 can make progress, and then try the fixup again.

Gratian provided a great analysis, traces and a reproducer. The analysis is
to the point, but it confused the hell out of that tglx dude who had to
page in all the futex horrors again. Condensed version is above.

[ tglx: Wrote comment and changelog ]

Fixes: c1e2f0eaf0 ("futex: Avoid violating the 10th rule of futex")
Reported-by: Gratian Crisan <[email protected]>
Signed-off-by: Mike Galbraith <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]

kernel/futex.c

@@ -2380,10 +2380,22 @@ retry:
 		}
 
 		/*
-		 * Since we just failed the trylock; there must be an owner.
+		 * The trylock just failed, so either there is an owner or
+		 * there is a higher priority waiter than this one.
 		 */
 		newowner = rt_mutex_owner(&pi_state->pi_mutex);
-		BUG_ON(!newowner);
+		/*
+		 * If the higher priority waiter has not yet taken over the
+		 * rtmutex then newowner is NULL. We can't return here with
+		 * that state because it's inconsistent vs. the user space
+		 * state. So drop the locks and try again. It's a valid
+		 * situation and not any different from the other retry
+		 * conditions.
+		 */
+		if (unlikely(!newowner)) {
+			err = -EAGAIN;
+			goto handle_err;
+		}
 	} else {
 		WARN_ON_ONCE(argowner != current);
 		if (oldowner == current) {