[PATCH 2/4] rtmutex: add WARN_ON if a task attempts to block on two locks

[Darren Hart]

rtmutex proxy locking complicates the logic a bit and opens up
the possibility for a task to wake and attempt to take another
sleeping lock without knowing it has been enqueued on another
lock already. Add a WARN_ON to catch this scenario early.

V2: use a WARN_ON instead of a BUG_ON per Steven's request.

Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Cc: John Kacur <jkacur@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
---
 kernel/rtmutex.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index baac7d9..5262d0f 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -459,6 +459,9 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock,
 		top_waiter = rt_mutex_top_waiter(lock);
 	plist_add(&waiter->list_entry, &lock->wait_list);
 
+	/* Tasks can only block on one lock at a time. */
+	WARN_ON(task->pi_blocked_on != NULL);
+
 	task->pi_blocked_on = waiter;
 
 	raw_spin_unlock(&task->pi_lock);
-- 
1.7.0.4

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