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include/linux/cache.h: expand documentation over __read_mostly

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__read_mostly can easily be misused by folks, its not meant for just
read-only data.  There are performance reasons for using it, but we also
don't provide any guidance about its use.  Provide a bit more guidance
over its use.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Wilcox <willy@infradead.org>
Link: http://lkml.kernel.org/r/20200507161424.2584-1-mcgrof@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Luis Chamberlain 2020-06-08 21:35:07 -07:00 committed by Linus Torvalds
parent 2a71e81d32
commit 4fa7252338
1 changed files with 8 additions and 2 deletions
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include/linux/cache.h
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@ -15,8 +15,14 @@
/*
* __read_mostly is used to keep rarely changing variables out of frequently
* updated cachelines. If an architecture doesn't support it, ignore the
* hint.
* updated cachelines. Its use should be reserved for data that is used
* frequently in hot paths. Performance traces can help decide when to use
* this. You want __read_mostly data to be tightly packed, so that in the
* best case multiple frequently read variables for a hot path will be next
* to each other in order to reduce the number of cachelines needed to
* execute a critical path. We should be mindful and selective of its use.
* ie: if you're going to use it please supply a *good* justification in your
* commit log
*/
#ifndef __read_mostly
#define __read_mostly