Messages in this thread Patch in this message | | | Date | Tue, 25 Apr 2006 16:41:05 -0000 | From | Thomas Gleixner <> | Subject | [patch 1/4] rtmutex: Remove buggy BUG_ON in PI boosting code |
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From: Steven Rostedt <rostedt@goodmis.org>
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@goodmis.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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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