Re: [PATCH RT v2] futex/rtmutex: Cure RT double blocking issue

[Steven Rostedt]

On Thu, 11 May 2017 17:20:54 +0200
Sebastian Sewior <bigeasy@xxxxxxxxxxxxx> wrote:

This is the same patch that Thomas wrote, right? Shouldn't this start
with:

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

?

-- Steve


> 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@xxxxxxxxxxxxx>
> Reported-by: Engleder Gerhard <eg@xxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>  v1âv2: 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;