The current deadlock detection logic does not work reliably due to the
following early exit path:
/*
* Drop out, when the task has no waiters. Note,
* top_waiter can be NULL, when we are in the deboosting
* mode!
*/
if (top_waiter && (!task_has_pi_waiters(task) ||
top_waiter != task_top_pi_waiter(task)))
goto out_unlock_pi;
So this not only exits when the task has no waiters, it also exits
unconditionally when the current waiter is not the top priority waiter
of the task.
So in a nested locking scenario, it might abort the lock chain walk
and therefor miss a potential deadlock.
Simple fix: Continue the chain walk, when deadlock detection is
enabled.
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
---
kernel/locking/rtmutex.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)
Index: linux-2.6/kernel/locking/rtmutex.c
===================================================================
--- linux-2.6.orig/kernel/locking/rtmutex.c
+++ linux-2.6/kernel/locking/rtmutex.c
@@ -343,16 +343,24 @@ static int rt_mutex_adjust_prio_chain(st
* top_waiter can be NULL, when we are in the deboosting
* mode!
*/
- if (top_waiter && (!task_has_pi_waiters(task) ||
- top_waiter != task_top_pi_waiter(task)))
- goto out_unlock_pi;
+ if (top_waiter) {
+ if (!task_has_pi_waiters(task))
+ goto out_unlock_pi;
+
+ if (top_waiter != task_top_pi_waiter(task)) {
+ if (!detect_deadlock)
+ goto out_unlock_pi;
+ }
+ }
/*
* When deadlock detection is off then we check, if further
* priority adjustment is necessary.
*/
- if (!detect_deadlock && waiter->prio == task->prio)
- goto out_unlock_pi;
+ if (waiter->prio == task->prio) {
+ if (!detect_deadlock)
+ goto out_unlock_pi;
+ }
lock = waiter->lock;
if (!raw_spin_trylock(&lock->wait_lock)) {
@@ -527,6 +535,10 @@ static int task_blocks_on_rt_mutex(struc
unsigned long flags;
int chain_walk = 0, res;
+ /* Early deadlock detection */
+ if (detect_deadlock && owner == task)
+ return -EDEADLK;
+
raw_spin_lock_irqsave(&task->pi_lock, flags);
__rt_mutex_adjust_prio(task);
waiter->task = task;
--
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