locking/rtmutex: Avoid a NULL pointer dereference on deadlock

With task_blocks_on_rt_mutex() returning early -EDEADLK we never
add the waiter to the waitqueue. Later, we try to remove it via
remove_waiter() and go boom in rt_mutex_top_waiter() because
rb_entry() gives a NULL pointer.

( Tested on v3.18-RT where rtmutex is used for regular mutex and I
  tried to get one twice in a row. )

Not sure when this started but I guess 397335f004f4 ("rtmutex: Fix
deadlock detector for real") or commit 3d5c9340d194 ("rtmutex:
Handle deadlock detection smarter").

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org> # for v3.16 and later kernels
Link: http://lkml.kernel.org/r/1424187823-19600-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 3059bc2f022daa6e4d8d976c39a7d8a8f546d813..e16e5542bf13f381a36c304b7e6b1ff9b66aaea0 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1193,7 +1193,8 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
                ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
 
        if (unlikely(ret)) {
-               remove_waiter(lock, &waiter);
+               if (rt_mutex_has_waiters(lock))
+                       remove_waiter(lock, &waiter);
                rt_mutex_handle_deadlock(ret, chwalk, &waiter);
        }