lkml.org 
[lkml]   [2014]   [May]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[patch 1/6] rtmutex: Fix deadlock detector for real
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 <tglx@linutronix.de>
Cc: stable@vger.kernel.org
---
kernel/locking/rtmutex.c | 27 +++++++++++++++++++++------
1 file changed, 21 insertions(+), 6 deletions(-)

Index: tip/kernel/locking/rtmutex.c
===================================================================
--- tip.orig/kernel/locking/rtmutex.c
+++ tip/kernel/locking/rtmutex.c
@@ -343,16 +343,22 @@ 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 (!detect_deadlock && top_waiter != task_top_pi_waiter(task))
+ 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)) {
@@ -361,7 +367,12 @@ static int rt_mutex_adjust_prio_chain(st
goto retry;
}

- /* Deadlock detection */
+ /*
+ * Deadlock detection. If the lock is the same as the original
+ * lock which caused us to walk the lock chain or if the
+ * current lock is owned by the task which initiated the chain
+ * walk, we detected a deadlock.
+ */
if (lock == orig_lock || rt_mutex_owner(lock) == top_task) {
debug_rt_mutex_deadlock(deadlock_detect, orig_waiter, lock);
raw_spin_unlock(&lock->wait_lock);
@@ -527,6 +538,10 @@ 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;



\
 
 \ /
  Last update: 2014-05-22 06:01    [W:0.141 / U:0.844 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site