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jump_label: Add comment about initialization order for anonymous unions

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Commit 3821fd35b5 ("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@oracle.com>
Suggested-by: Chris Mason <clm@fb.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware) 2017-03-02 17:28:45 -05:00
parent cd8d860dcc
commit b17ef2ed62
1 changed files with 7 additions and 0 deletions
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7
include/linux/jump_label.h
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@ -90,6 +90,13 @@ extern bool static_key_initialized;
struct static_key { struct static_key {
atomic_t enabled; 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 * bit 0 => 1 if key is initially true
* 0 if initially false * 0 if initially false
* bit 1 => 1 if points to struct static_key_mod * bit 1 => 1 if points to struct static_key_mod