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jump_label: Add some documentation

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akpm figured we could do with a blub explaining what static_branch()
is and why it lives...

Grumpily-requested-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jason Baron <jbaron@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-h02wu6kabpoojxf03wke704k@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2012-01-26 13:32:15 +01:00 committed by Ingo Molnar
parent 44a6839711
commit efb3040d48
1 changed files with 32 additions and 0 deletions
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32
include/linux/jump_label.h
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@ -1,6 +1,38 @@
#ifndef _LINUX_JUMP_LABEL_H
#define _LINUX_JUMP_LABEL_H
/*
* Jump label support
*
* Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
* Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
*
* Jump labels provide an interface to generate dynamic branches using
* self-modifying code. Assuming toolchain and architecture support the result
* of a "if (static_branch(&key))" statement is a unconditional branch (which
* defaults to false - and the true block is placed out of line).
*
* However at runtime we can change the 'static' branch target using
* jump_label_{inc,dec}(). These function as a 'reference' count on the key
* object and for as long as there are references all branches referring to
* that particular key will point to the (out of line) true block.
*
* Since this relies on modifying code the jump_label_{inc,dec}() functions
* must be considered absolute slow paths (machine wide synchronization etc.).
* OTOH, since the affected branches are unconditional their runtime overhead
* will be absolutely minimal, esp. in the default (off) case where the total
* effect is a single NOP of appropriate size. The on case will patch in a jump
* to the out-of-line block.
*
* When the control is directly exposed to userspace it is prudent to delay the
* decrement to avoid high frequency code modifications which can (and do)
* cause significant performance degradation. Struct jump_label_key_deferred and
* jump_label_dec_deferred() provide for this.
*
* Lacking toolchain and or architecture support, it falls back to a simple
* conditional branch.
*/
#include <linux/types.h>
#include <linux/compiler.h>
#include <linux/workqueue.h>