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Date: Tue, 30 Sep 2014 00:13:44 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, torvalds@linux-foundation.org, tglx@linutronix.de,
        ilya.dryomov@inktank.com, umgwanakikbuti@gmail.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 10/11] sched: Debug nested sleeps
Message-ID: <20140929221344.GB12112@redhat.com>
References: <20140924081845.572814794@infradead.org> <20140924082242.591637616@infradead.org>
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On 09/24, Peter Zijlstra wrote:
>
> +#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> +
> +#define __set_task_state(tsk, state_value)			\
> +	do {							\
> +		(tsk)->task_state_change = _THIS_IP_;		\
> +		(tsk)->state = (state_value);			\
> +	} while (0)

...

> @@ -7143,6 +7143,19 @@ void __might_sleep(const char *file, int
>  {
>  	static unsigned long prev_jiffy;	/* ratelimiting */
>
> +	/*
> +	 * Blocking primitives will set (and therefore destroy) current->state,
> +	 * since we will exit with TASK_RUNNING make sure we enter with it,
> +	 * otherwise we will destroy state.
> +	 */
> +	if (WARN(current->state != TASK_RUNNING,
> +			"do not call blocking ops when !TASK_RUNNING; "
> +			"state=%lx set at [<%p>] %pS\n",
> +			current->state,
> +			(void *)current->task_state_change,
> +			(void *)current->task_state_change))
> +		__set_current_state(TASK_RUNNING);

Question: now that we have ->task_state_change, perhaps it makes sense
to redefine fixup_sleep()

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

and make the WARN() above depend on task_state_change != 0 ?

This is minor, but this way CONFIG_DEBUG_ATOMIC_SLEEP will not imply
a subtle behavioural change.

Oleg.