From: Thomas Gleixner Date: Tue, 9 May 2017 17:11:10 +0200 Subject: [PATCH] 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 Reported-by: Engleder Gerhard Signed-off-by: Sebastian Andrzej Siewior --- kernel/locking/rtmutex.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -2407,6 +2407,7 @@ 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); @@ -2418,6 +2419,24 @@ int rt_mutex_wait_proxy_lock(struct rt_m * have to fix that up. */ fixup_rt_mutex_waiters(lock); + /* + * 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;