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 <[email protected]> Reported-by: Engleder Gerhard <[email protected]> Signed-off-by: Sebastian Andrzej Siewior <[email protected]> --- v1…v2: reset ->pi_blocked_on only in the error case. kernel/locking/rtmutex.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 314fc65a35b1..4675f1197f33 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -2400,6 +2400,7 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock, struct hrtimer_sleeper *to, struct rt_mutex_waiter *waiter) { + struct task_struct *tsk = current; int ret; raw_spin_lock_irq(&lock->wait_lock); @@ -2409,6 +2410,24 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock, /* 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. + */ + if (ret) { + 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; -- 2.11.0
