[tip:sched/core] sched/wait: Explain the shadowing and type inconsistencies

[tip:sched/core] sched/wait: Explain the shadowing and type inconsistencies

[tip-bot for Peter Zijlstra]

Commit-ID:  27e4f9d0012a9bb7011aade862f08679d2921ab0
Gitweb:     http://git.kernel.org/tip/27e4f9d0012a9bb7011aade862f08679d2921ab0
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 9 Apr 2014 12:50:34 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 18 Apr 2014 12:07:22 +0200

sched/wait: Explain the shadowing and type inconsistencies

Stick in a comment before someone else tries to fix the sparse warning
this generates.

Requested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-o2ro6f3vkxklni0bc8f7m68s@git.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/wait.h | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 559044c..2b563a1 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -191,11 +191,23 @@ wait_queue_head_t *bit_waitqueue(void *, int);
 	(!__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)							\