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Subject[PATCH RT] futex/rtmutex: Cure RT double blocking issue
RT has a problem when the wait on a futex/rtmutex got interrupted by a
timeout or a signal. task->pi_blocked_on is still set when returning from
rt_mutex_wait_proxy_lock(). The task must acquire the hash bucket lock
after this.

If the hash bucket lock is contended then the
BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in
task_blocks_on_rt_mutex() will trigger.

This can be avoided by clearing task->pi_blocked_on in the return path of
rt_mutex_wait_proxy_lock() which removes the task from the boosting chain
of the rtmutex. That's correct because the task is not longer blocked on
it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Engleder Gerhard <eg@keba.com>
---
kernel/locking/rtmutex.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -2380,6 +2380,7 @@ int rt_mutex_wait_proxy_lock(struct rt_m
struct hrtimer_sleeper *to,
struct rt_mutex_waiter *waiter)
{
+ struct task_struct *tsk = current;
int ret;

raw_spin_lock_irq(&lock->wait_lock);
@@ -2389,6 +2390,22 @@ int rt_mutex_wait_proxy_lock(struct rt_m
/* sleep on the mutex */
ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter, NULL);

+ /*
+ * RT has a problem here when the wait got interrupted by a timeout
+ * or a signal. task->pi_blocked_on is still set. The task must
+ * acquire the hash bucket lock when returning from this function.
+ *
+ * If the hash bucket lock is contended then the
+ * BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in
+ * task_blocks_on_rt_mutex() will trigger. This can be avoided by
+ * clearing task->pi_blocked_on which removes the task from the
+ * boosting chain of the rtmutex. That's correct because the task
+ * is not longer blocked on it.
+ */
+ raw_spin_lock(&tsk->pi_lock);
+ tsk->pi_blocked_on = NULL;
+ raw_spin_unlock(&tsk->pi_lock);
+
raw_spin_unlock_irq(&lock->wait_lock);

return ret;
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