linux-kernel - Re: [PATCH 10/11] sched: Debug nested sleeps

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Date:	Wed, 1 Oct 2014 20:35:49 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, torvalds@...ux-foundation.org,
	tglx@...utronix.de, ilya.dryomov@...tank.com,
	umgwanakikbuti@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/11] sched: Debug nested sleeps

On 10/01, Peter Zijlstra wrote:
>
> On Tue, Sep 30, 2014 at 11:47:32PM +0200, Oleg Nesterov wrote:
>
> > > > This is minor, but this way CONFIG_DEBUG_ATOMIC_SLEEP will not imply
> > > > a subtle behavioural change.
> > >
> > > You mean the __set_current_state() that's extra?
> >
> > Yes, and note that it only does __set_current_state(RUNNING) if
> > CONFIG_DEBUG_ATOMIC_SLEEP. This means that disabling/enabling this
> > option can, silently hide/uncover a bug.
> >
> > > I would actually argue
> > > to keep that since it makes the 'problem' much worse.
> >
> > OK, I won't insist, but could you explain why the suggested change can
> > make the problem (and which problem ;) worse?
>
> Sure, so the trivial problem is not actually going to sleep in the outer
> wait primitive because the inner wait primitive reset ->state to
> TASK_RUNNING.

But this means that fixup_sleep() must not be used?

> So by always setting the ->state to TASK_RUNNING it never goes to sleep
> and it'll revert to spinning,

But I tried to suggest to not set TASK_RUNNING?


Peter, I am sorry for wasting your time, this is really minor, but still
I'd like to understand.

Let me try again. With this series we have

	#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
	#define fixup_sleep()	__set_current_state(TASK_RUNNING)
	#else
	#define fixup_sleep()	do { } while (0)
	#endif

and this means that we do not need __set_current_state(RUNNING) for
correctness, just we want to shut up the warning in __might_sleep().
This is fine (and the self-documenting helper is nice), but this means
that CONFIG_DEBUG_ATOMIC_SLEEP adds a subtle difference.

For example, let's suppose that we do not have 01/11 which fixes
mutex_lock(). Then this code

	set_current_state(TASK_UNINTERRUPTIBLE);
	...
	fixup_sleep();
	...
	mutex_lock(some_mutex);

can hang, but only if !CONFIG_DEBUG_ATOMIC_SLEEP.

So perhaps it makes sense to redefine it

	#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
	#define fixup_sleep()	(current->task_state_change = 0)
	#else
	#define fixup_sleep()	do { } while (0)
	#endif

and change __might_sleep()

	-	if (WARN(current->state != TASK_RUNNING,
	+	if (WARN(current->state != TASK_RUNNING && current->task_state_change != 0,

?

Oleg.

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