| From: Thomas Gleixner <tglx@linutronix.de> |
| 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 <tglx@linutronix.de> |
| Reported-by: Engleder Gerhard <eg@keba.com> |
| Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> |
| --- |
| kernel/locking/rtmutex.c | 19 +++++++++++++++++++ |
| 1 file changed, 19 insertions(+) |
| |
| --- a/kernel/locking/rtmutex.c |
| +++ b/kernel/locking/rtmutex.c |
| @@ -2406,6 +2406,7 @@ 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); |
| @@ -2417,6 +2418,24 @@ 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; |