[patch 1/4] rtmutex: Remove buggy BUG_ON in PI boosting code

[Thomas Gleixner]

From: Steven Rostedt <rostedt@xxxxxxxxxxx>

The condition in that particular BUG_ON can legitimately be the
case, if you have processes A, B, C, D, and E holding the
following locks in this scenario:

 L1 <=blocks= A
               <=owns= L2 <=blocks= B <=owns= L4 <=blocks= D
               <=owns= L3 <=blocks= C <=owns= L5 <=blocks= E

Where the priorities of these tasks are

    B,C < A < D = E

B and C are less than A and A is less than D and E where D and E are
equal (actually it probably works when D and E are not equal too).

As D and E climb the chain, there's a very slight race condition that
could allow for the condition in the offending BUG_ON to be true.

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>

 kernel/rtmutex.c |    4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

Index: linux-2.6.17-rc1-mm3/kernel/rtmutex.c
===================================================================
--- linux-2.6.17-rc1-mm3.orig/kernel/rtmutex.c
+++ linux-2.6.17-rc1-mm3/kernel/rtmutex.c
@@ -209,10 +209,8 @@ static int rt_mutex_adjust_prio_chain(ta
 	 * When deadlock detection is off then we check, if further
 	 * priority adjustment is necessary.
 	 */
-	if (!detect_deadlock && waiter->list_entry.prio == task->prio) {
-		BUG_ON(waiter->pi_list_entry.prio != waiter->list_entry.prio);
+	if (!detect_deadlock && waiter->list_entry.prio == task->prio)
 		goto out_unlock_pi;
-	}
 
 	lock = waiter->lock;
 	if (!spin_trylock(&lock->wait_lock)) {

--

-
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