Subject: Re: Regression with wait_event_timeout in next-20140226

From: Peter Zijlstra <[email-masked]>
Date: Wed, 9 Apr 2014 13:16:38 +0200

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 <[email-masked]>
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 <[email-masked]>
Signed-off-by: Peter Zijlstra <[email-masked]>
Link: http://lkml.kernel.org/n/[email-masked]
---
 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)							\