jump_label: Add some documentation

#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>