linux-kernel - Re: Regression with wait_event

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Date:	Wed, 9 Apr 2014 12:05:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Gregory CLEMENT <gregory.clement@...e-electrons.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-next@...r.kernel.org,
	Johannes Berg <johannes.berg@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Regression with wait_event_timeout in next-20140226

On Wed, 9 Apr 2014 13:16:38 +0200 Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Feb 26, 2014 at 02:25:34PM -0800, Andrew Morton wrote:
> > Is there anything we can do to make all this clearer?  Simply using a
> > distinctive variable name ("__wait_var__"?) in place of __ret (and
> > documenting it) would help a lot.
> > 
> > Some __ret's are long and some are int.  Maybe that's a glitch, maybe
> > it's because some __ret's are used for inter-macro communications and
> > some are not, which just makes things worse.
> > 
> > I started to do a patch, got all confused and gave up.  We've made
> > quite a tangly mess in there, alas.
> 
> Something like so?
> 
> ---
> Subject: wait: Explain the shadowing and type inconsistencies
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Wed Apr  9 12:50:34 CEST 2014
> 
> Stick in a comment before someone else tries to fix the sparse warning
> this generates.
> 
> Requested-by: Andrew Morton <akpm@...ux-foundation.org>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Link: http://lkml.kernel.org/n/tip-o2ro6f3vkxklni0bc8f7m68s@git.kernel.org
> ---
>  include/linux/wait.h |   14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -191,11 +191,23 @@ wait_queue_head_t *bit_waitqueue(void *,
>  	(!__builtin_constant_p(state) ||				\
>  		state == TASK_INTERRUPTIBLE || state == TASK_KILLABLE)	\
>  
> +/*
> + * The below macro ___wait_event() has an explicit shadow of the __ret
> + * variable when used from the wait_event_*() macros.
> + *
> + * This is so that both can use the ___wait_cond_timeout() construct
> + * to wrap the condition.
> + *
> + * The type inconsistency of the wait_event_*() __ret variable is also
> + * on purpose; we use long where we can return timeout values and int
> + * otherwise.
> + */
> +
>  #define ___wait_event(wq, condition, state, exclusive, ret, cmd)	\
>  ({									\
>  	__label__ __out;						\
>  	wait_queue_t __wait;						\
> -	long __ret = ret;						\
> +	long __ret = ret;	/* explicit shadow */			\
>  									\
>  	INIT_LIST_HEAD(&__wait.task_list);				\
>  	if (exclusive)							\

Looks nice to me, thanks.
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