From 7ffb28ebaaf76cdf3c602368f344e086358afb0f Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Tue, 17 Feb 2015 16:18:05 +0100 Subject: [PATCH] locking/rt-mutex: 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 397335f00 ("rtmutex: Fix deadlock detector for real") or commit 3d5c9340 ("rtmutex: Handle deadlock detection smarter"). Cc: stable-rt@vger.kernel.org Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Steven Rostedt --- kernel/locking/rtmutex.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index a0d9eadcfe02..ec72272ab73a 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1697,7 +1697,8 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state, set_current_state(TASK_RUNNING); if (unlikely(ret)) { - remove_waiter(lock, &waiter); + if (rt_mutex_has_waiters(lock)) + remove_waiter(lock, &waiter); rt_mutex_handle_deadlock(ret, chwalk, &waiter); } else if (ww_ctx) { ww_mutex_account_lock(lock, ww_ctx);