locking/rtmutex: Fix ww_mutex deadlock check
Dan reported that rt_mutex_adjust_prio_chain() can be called with
.orig_waiter == NULL however commit a055fcc132d4 ("locking/rtmutex: Return
success on deadlock for ww_mutex waiters") unconditionally dereferences it.
Since both call-sites that have .orig_waiter == NULL don't care for the
return value, simply disable the deadlock squash by adding the NULL check.
Notably, both callers use the deadlock condition as a termination condition
for the iteration; once detected, it is sure that (de)boosting is done.
Arguably step [3] would be a more natural termination point, but it's
dubious whether adding a third deadlock detection state would improve the
code.
Fixes: a055fcc132d4 ("locking/rtmutex: Return success on deadlock for ww_mutex waiters")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/YS9La56fHMiCCo75@hirez.programming.kicks-ass.net
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 8eabdc7..6bb116c 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -753,7 +753,7 @@
* other configuration and we fail to report; also, see
* lockdep.
*/
- if (IS_ENABLED(CONFIG_PREEMPT_RT) && orig_waiter->ww_ctx)
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && orig_waiter && orig_waiter->ww_ctx)
ret = 0;
raw_spin_unlock(&lock->wait_lock);
