linux-kernel - Re: [tip:sched/core] sched: Fix ancient race in do

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Date:	Sun, 29 Jan 2012 19:59:49 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, y-goto@...fujitsu.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Fix ancient race in do_exit()

On 01/29, Linus Torvalds wrote:
>
> But yes, if you're talking about TASK_UNINTERRUPTIBLE, we do need to
> just remove the setting of that entirely. It needs to be set *before*
> adding us to the list, not after. That's just a bug - we get woken up
> when we've been given the lock.

Yes, I think this should work although I am not familiar with this code.

If we remove set_task_state() from the main waiting loop we can never race
with __rwsem_do_wake()->try_to_wake_up() seeing us in UNINTERRUPTIBLE state.
rwsem_down_failed_common() simply can't return until UNINTERRUPTIBLE->RUNNING
transition is finished (__rwsem_do_wake does wakeup first).

And since we do not play with current->state after spin_unlock(), it is
fine to "race" with waiter->task clearing, just we can do the unnecessary
but harmless schedule() in TASK_RUNNING.

> So it may be completely and utterly broken for some subtle reason,

Well, what about another spurious wakeup from somewhere? In this case
rwsem_down_failed_common() will do a busy-wait loop.

> @@ -92,10 +92,9 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
>  	 */
>  	list_del(&waiter->list);
>  	tsk = waiter->task;
> +	wake_up_process(tsk);
>  	smp_mb();
>  	waiter->task = NULL;

OK, now I understand why do we need "clear after wakeup".

But then I don't really understand this mb, perhaps wmb() is enough?
Afaics we only need to ensure we change waiter->task after changing
task's state.

OTOH,

> @@ -183,7 +181,6 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
>  	raw_spin_lock_irq(&sem->wait_lock);
>  	waiter.task = tsk;
>  	waiter.flags = flags;
> -	get_task_struct(tsk);
>
>  	if (list_empty(&sem->wait_list))
>  		adjustment += RWSEM_WAITING_BIAS;
> @@ -211,11 +208,8 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
>  		if (!waiter.task)
>  			break;
>  		schedule();
> -		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
>  	}
>
> -	tsk->state = TASK_RUNNING;
> -
>  	return sem;
>  }

Suppose that this task does tsk->state = TASK_WHATEVER after that.
It seems that we need mb() before return, otherwise the next ->state
change can be reordered with "if (!waiter.task)" above. Or not?

Oleg.

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