Subject: Re: [PATCH 6/7 linux-next] wan: cosa: replace current->state by set_current_state()

From: Peter Zijlstra <[email-masked]>
Date: Fri, 20 Feb 2015 21:02:26 +0100

On Fri, Feb 20, 2015 at 11:31:53AM -0800, Eric Dumazet wrote:
> On Fri, 2015-02-20 at 19:58 +0100, Peter Zijlstra wrote:

> Maybe this might be self documented in set_current_state(),
> as we have about 120 calls to __set_current_state(TASK_RUNNING)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 41c60e5302d7..26133da6445e 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -275,7 +275,11 @@ extern char ___assert_task_state[1 - 2*!!(
>  #define set_current_state(state_value)				\
>  	do {							\
>  		current->task_state_change = _THIS_IP_;		\
> -		set_mb(current->state, (state_value));		\
> +		if (__builtin_constant_p(state_value) &&	\
> +		    (state_value) == TASK_RUNNING)		\
> +			current->state = (state_value);		\
> +		else						\
> +			set_mb(current->state, (state_value));	\
>  	} while (0)

lkml.kernel.org/r/[email-masked]

The problem is that there _might_ be someone relying on that barrier.

Its (very) unlikely, but you don't want to risk subtle borkage just
because. And I'm too lazy to go audit all of them :/