locking/rtmutex: Squash self-deadlock check for ww_rt_mutex.

Similar to the issues in commits:

  6467822b8cc9 ("locking/rtmutex: Prevent spurious EDEADLK return caused by ww_mutexes")
  a055fcc132d4 ("locking/rtmutex: Return success on deadlock for ww_mutex waiters")

ww_rt_mutex_lock() should not return EDEADLK without first going through
the __ww_mutex logic to set the required state. In fact, the chain-walk
can deal with the spurious cycles (per the above commits) this check
warns about and is trying to avoid.

Therefore ignore this test for ww_rt_mutex and simply let things fall
in place.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20211129174654.668506-4-bigeasy@linutronix.de

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 0c6a48dfcecb346e5fffbabbe9ed9655ce488a4f..f89620852774dbbab5020b68494120d822e34cad 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1103,8 +1103,11 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
         * the other will detect the deadlock and return -EDEADLOCK,
         * which is wrong, as the other waiter is not in a deadlock
         * situation.
+        *
+        * Except for ww_mutex, in that case the chain walk must already deal
+        * with spurious cycles, see the comments at [3] and [6].
         */
-       if (owner == task)
+       if (owner == task && !(build_ww_mutex() && ww_ctx))
                return -EDEADLK;
 
        raw_spin_lock(&task->pi_lock);