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jump label, locking/static_keys: Update docs

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Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: benh@kernel.crashing.org
Cc: bp@alien8.de
Cc: davem@davemloft.net
Cc: ddaney@caviumnetworks.com
Cc: heiko.carstens@de.ibm.com
Cc: linux-kernel@vger.kernel.org
Cc: liuj97@gmail.com
Cc: luto@amacapital.net
Cc: michael@ellerman.id.au
Cc: rabin@rab.in
Cc: ralf@linux-mips.org
Cc: rostedt@goodmis.org
Cc: vbabka@suse.cz
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/6b50f2f6423a2244f37f4b1d2d6c211b9dcdf4f8.1438227999.git.jbaron@akamai.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Jason Baron 2015-07-30 03:59:48 +00:00 • committed by Ingo Molnar
parent 2bf9e0ab08
commit 412758cb26
2 changed files with 99 additions and 69 deletions
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67
include/linux/jump_label.h
View file

@ -7,17 +7,52 @@
* Copyright (C) 2009-2012 Jason Baron <jbaron@redhat.com>
* Copyright (C) 2011-2012 Peter Zijlstra <pzijlstr@redhat.com>
*
* DEPRECATED API:
*
* The use of 'struct static_key' directly, is now DEPRECATED. In addition
* static_key_{true,false}() is also DEPRECATED. IE DO NOT use the following:
*
* struct static_key false = STATIC_KEY_INIT_FALSE;
* struct static_key true = STATIC_KEY_INIT_TRUE;
* static_key_true()
* static_key_false()
*
* The updated API replacements are:
*
* DEFINE_STATIC_KEY_TRUE(key);
* DEFINE_STATIC_KEY_FALSE(key);
* static_key_likely()
* statick_key_unlikely()
*
* Jump labels provide an interface to generate dynamic branches using
* self-modifying code. Assuming toolchain and architecture support, the result
* of a "if (static_key_false(&key))" statement is an unconditional branch (which
* defaults to false - and the true block is placed out of line).
* self-modifying code. Assuming toolchain and architecture support, if we
* define a "key" that is initially false via "DEFINE_STATIC_KEY_FALSE(key)",
* an "if (static_branch_unlikely(&key))" statement is an unconditional branch
* (which defaults to false - and the true block is placed out of line).
* Similarly, we can define an initially true key via
* "DEFINE_STATIC_KEY_TRUE(key)", and use it in the same
* "if (static_branch_unlikely(&key))", in which case we will generate an
* unconditional branch to the out-of-line true branch. Keys that are
* initially true or false can be using in both static_branch_unlikely()
* and static_branch_likely() statements.
*
* However at runtime we can change the branch target using
* static_key_slow_{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.
* At runtime we can change the branch target by setting the key
* to true via a call to static_branch_enable(), or false using
* static_branch_disable(). If the direction of the branch is switched by
* these calls then we run-time modify the branch target via a
* no-op -> jump or jump -> no-op conversion. For example, for an
* initially false key that is used in an "if (static_branch_unlikely(&key))"
* statement, setting the key to true requires us to patch in a jump
* to the out-of-line of true branch.
*
* Since this relies on modifying code, the static_key_slow_{inc,dec}() functions
* In addtion to static_branch_{enable,disable}, we can also reference count
* the key or branch direction via static_branch_{inc,dec}. Thus,
* static_branch_inc() can be thought of as a 'make more true' and
* static_branch_dec() as a 'make more false'. The inc()/dec()
* interface is meant to be used exclusively from the inc()/dec() for a given
* key.
*
* Since this relies on modifying code, the branch modifying 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
@ -29,20 +64,10 @@
* cause significant performance degradation. Struct static_key_deferred and
* static_key_slow_dec_deferred() provide for this.
*
* Lacking toolchain and or architecture support, jump labels fall back to a simple
* conditional branch.
* Lacking toolchain and or architecture support, static keys fall back to a
* simple conditional branch.
*
* struct static_key my_key = STATIC_KEY_INIT_TRUE;
*
* if (static_key_true(&my_key)) {
* }
*
* will result in the true case being in-line and starts the key with a single
* reference. Mixing static_key_true() and static_key_false() on the same key is not
* allowed.
*
* Not initializing the key (static data is initialized to 0s anyway) is the
* same as using STATIC_KEY_INIT_FALSE.
* Additional babbling in: Documentation/static-keys.txt
*/
#if defined(CC_HAVE_ASM_GOTO) && defined(CONFIG_JUMP_LABEL)