linux-kernel - Re: [PATCH] sched: Avoid that __wait_on_bit

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Date:	Wed, 3 Aug 2016 20:11:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	"mingo@...nel.org" <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Neil Brown <neilb@...e.de>,
	Michael Shaver <jmshaver@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

On Wed, Aug 03, 2016 at 09:35:03AM -0700, Bart Van Assche wrote:
> If try_to_wakeup() reads the task state before abort_exclusive_wait()
> sets the task state and if autoremove_wake_function() is called after
> abort_exclusive_wait() has removed a task from a wait list then the
> cascading mechanism for exclusive wakeups in abort_exclusive_wait()
> won't be triggered. Avoid this by serializing the task state change
> in abort_exclusive_wait() and try_to_wakeup().

I'm dense.. what!?

	CPU0			CPU1			CPU2

	

				__lock_page_killable()
				  __wait_on_bit_lock()
				    bit_wait_io()
				      schedule()
	__wake_up_bit()
	  __wake_up(.nr_exclusive=1)
	    spin_lock(&q->lock)
	    __wake_up_common()
	      autoremove_wake_func()
	        try_to_wake_up(p, TASK_NORMAL)
		list_del_init(&wait->task_list)
	    spin_unlock(&q->lock)

							complete_signal(p)
							  signal_wake_up(p, 1)
							    sigaddset(&p->pending.signal, SIGKILL)
							    try_to_wake_up(p, TASK_WAKEKILL)

				      if (signal_pending_state(TASK_KILLABLE))
				        return -EINTR;
				    abort_exclusive_wait()
				      __set_current_state(RUNNING)
				      spin_lock(q->lock)
				      if (!list_empty()) /* empty */
				      else if (waitqueue_active()) /* pending ? */
				        __wake_up_locked_key(q, mode, key)
				      spin_unlock(q->lock)


That seems to do the right thing, so clearly I misunderstand. Please
clarify.


> +++ b/kernel/sched/wait.c
> @@ -277,10 +277,17 @@ void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait,
>  			unsigned int mode, void *key)
>  {
>  	unsigned long flags;
> +	long wake_up;
> +
> +	/* Serialize against try_to_wake_up() */
> +	raw_spin_lock_irqsave(&current->pi_lock, flags);
> +	wake_up = current->state & (TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE);
> +	if (wake_up)
> +		__set_current_state(TASK_RUNNING);
> +	raw_spin_unlock_irqrestore(&current->pi_lock, flags);
>  
> -	__set_current_state(TASK_RUNNING);
>  	spin_lock_irqsave(&q->lock, flags);
> -	if (!list_empty(&wait->task_list))
> +	if (wake_up)
>  		list_del_init(&wait->task_list);
>  	else if (waitqueue_active(q))
>  		__wake_up_locked_key(q, mode, key);

That just feels wrong,.. very wrong.