linux-kernel - [PATCH] jump_label: Add comment about initialization order for anonymous unions

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Message-ID: <20170302173526.59988f17@gandalf.local.home>
Date:   Thu, 2 Mar 2017 17:35:26 -0500
From:   "Steven Rostedt (VMware)" <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Jason Baron <jbaron@...mai.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Chris Mason <clm@...com>
Subject: [PATCH] jump_label: Add comment about initialization order for
 anonymous unions


Commit 3821fd35b58d ("jump_label: Reduce the size of struct static_key")
broke old compilers that could not handle static initialization of anonymous
unions. Boris fixed it with a patch that added brackets around the static
initializer. But this creates a dependency between those initializers
and the structure's order of its fields. Document this dependency in
case new fields are added to struct static_key in the future.

Noted-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Suggested-by: Chris Mason <clm@...com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 include/linux/jump_label.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index a7f9011..28e04a33 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -90,6 +90,13 @@ extern bool static_key_initialized;
 struct static_key {
 	atomic_t enabled;
 /*
+ * Note:
+ *   To make anonymous unions work with old compilers, the static
+ *   initialization of them requires brackets. This creates a dependency
+ *   on the order of the struct with the initializers. If any fields
+ *   are added, STATIC_KEY_INIT_TRUE and STATIC_KEY_INIT_FALSE may need
+ *   to be modified.
+ *
  * bit 0 => 1 if key is initially true
  *	    0 if initially false
  * bit 1 => 1 if points to struct static_key_mod
-- 
2.9.3