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locking/static_keys: Add a new static_key interface

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There are various problems and short-comings with the current
static_key interface:

 - static_key_{true,false}() read like a branch depending on the key
   value, instead of the actual likely/unlikely branch depending on
   init value.

 - static_key_{true,false}() are, as stated above, tied to the
   static_key init values STATIC_KEY_INIT_{TRUE,FALSE}.

 - we're limited to the 2 (out of 4) possible options that compile to
   a default NOP because that's what our arch_static_branch() assembly
   emits.

So provide a new static_key interface:

  DEFINE_STATIC_KEY_TRUE(name);
  DEFINE_STATIC_KEY_FALSE(name);

Which define a key of different types with an initial true/false
value.

Then allow:

   static_branch_likely()
   static_branch_unlikely()

to take a key of either type and emit the right instruction for the
case.

This means adding a second arch_static_branch_jump() assembly helper
which emits a JMP per default.

In order to determine the right instruction for the right state,
encode the branch type in the LSB of jump_entry::key.

This is the final step in removing the naming confusion that has led to
a stream of avoidable bugs such as:

  a833581e37 ("x86, perf: Fix static_key bug in load_mm_cr4()")

... but it also allows new static key combinations that will give us
performance enhancements in the subsequent patches.

Tested-by: Rabin Vincent <rabin@rab.in> # arm
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> # ppc
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com> # s390
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: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2015-07-24 15:09:55 +02:00
committed by Ingo Molnar
parent 706249c222
commit 11276d5306
9 changed files with 299 additions and 45 deletions
Download Patch File Download Diff File Expand all files Collapse all files

149
include/linux/jump_label.h
View File

@@ -107,12 +107,12 @@ static inline int static_key_count(struct static_key *key)
static __always_inline bool static_key_false(struct static_key *key)
{
return arch_static_branch(key);
return arch_static_branch(key, false);
}
static __always_inline bool static_key_true(struct static_key *key)
{
return !static_key_false(key);
return !arch_static_branch(key, true);
}
extern struct jump_entry __start___jump_table[];
@@ -130,12 +130,12 @@ extern void static_key_slow_inc(struct static_key *key);
extern void static_key_slow_dec(struct static_key *key);
extern void jump_label_apply_nops(struct module *mod);
#define STATIC_KEY_INIT_TRUE ((struct static_key) \
#define STATIC_KEY_INIT_TRUE \
{ .enabled = ATOMIC_INIT(1), \
.entries = (void *)JUMP_TYPE_TRUE })
#define STATIC_KEY_INIT_FALSE ((struct static_key) \
.entries = (void *)JUMP_TYPE_TRUE }
#define STATIC_KEY_INIT_FALSE \
{ .enabled = ATOMIC_INIT(0), \
.entries = (void *)JUMP_TYPE_FALSE })
.entries = (void *)JUMP_TYPE_FALSE }
#else /* !HAVE_JUMP_LABEL */
@@ -183,10 +183,8 @@ static inline int jump_label_apply_nops(struct module *mod)
return 0;
}
#define STATIC_KEY_INIT_TRUE ((struct static_key) \
{ .enabled = ATOMIC_INIT(1) })
#define STATIC_KEY_INIT_FALSE ((struct static_key) \
{ .enabled = ATOMIC_INIT(0) })
#define STATIC_KEY_INIT_TRUE { .enabled = ATOMIC_INIT(1) }
#define STATIC_KEY_INIT_FALSE { .enabled = ATOMIC_INIT(0) }
#endif /* HAVE_JUMP_LABEL */
@@ -218,6 +216,137 @@ static inline void static_key_disable(struct static_key *key)
static_key_slow_dec(key);
}
/* -------------------------------------------------------------------------- */
/*
* Two type wrappers around static_key, such that we can use compile time
* type differentiation to emit the right code.
*
* All the below code is macros in order to play type games.
*/
struct static_key_true {
struct static_key key;
};
struct static_key_false {
struct static_key key;
};
#define STATIC_KEY_TRUE_INIT (struct static_key_true) { .key = STATIC_KEY_INIT_TRUE, }
#define STATIC_KEY_FALSE_INIT (struct static_key_false){ .key = STATIC_KEY_INIT_FALSE, }
#define DEFINE_STATIC_KEY_TRUE(name) \
struct static_key_true name = STATIC_KEY_TRUE_INIT
#define DEFINE_STATIC_KEY_FALSE(name) \
struct static_key_false name = STATIC_KEY_FALSE_INIT
#ifdef HAVE_JUMP_LABEL
/*
* Combine the right initial value (type) with the right branch order
* to generate the desired result.
*
*
* type\branch| likely (1) | unlikely (0)
* -----------+-----------------------+------------------
* | |
* true (1) | ... | ...
* | NOP | JMP L
* | <br-stmts> | 1: ...
* | L: ... |
* | |
* | | L: <br-stmts>
* | | jmp 1b
* | |
* -----------+-----------------------+------------------
* | |
* false (0) | ... | ...
* | JMP L | NOP
* | <br-stmts> | 1: ...
* | L: ... |
* | |
* | | L: <br-stmts>
* | | jmp 1b
* | |
* -----------+-----------------------+------------------
*
* The initial value is encoded in the LSB of static_key::entries,
* type: 0 = false, 1 = true.
*
* The branch type is encoded in the LSB of jump_entry::key,
* branch: 0 = unlikely, 1 = likely.
*
* This gives the following logic table:
*
* enabled type branch instuction
* -----------------------------+-----------
* 0 0 0 | NOP
* 0 0 1 | JMP
* 0 1 0 | NOP
* 0 1 1 | JMP
*
* 1 0 0 | JMP
* 1 0 1 | NOP
* 1 1 0 | JMP
* 1 1 1 | NOP
*
* Which gives the following functions:
*
* dynamic: instruction = enabled ^ branch
* static: instruction = type ^ branch
*
* See jump_label_type() / jump_label_init_type().
*/
extern bool ____wrong_branch_error(void);
#define static_branch_likely(x) \
({ \
bool branch; \
if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
branch = !arch_static_branch(&(x)->key, true); \
else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
branch = !arch_static_branch_jump(&(x)->key, true); \
else \
branch = ____wrong_branch_error(); \
branch; \
})
#define static_branch_unlikely(x) \
({ \
bool branch; \
if (__builtin_types_compatible_p(typeof(*x), struct static_key_true)) \
branch = arch_static_branch_jump(&(x)->key, false); \
else if (__builtin_types_compatible_p(typeof(*x), struct static_key_false)) \
branch = arch_static_branch(&(x)->key, false); \
else \
branch = ____wrong_branch_error(); \
branch; \
})
#else /* !HAVE_JUMP_LABEL */
#define static_branch_likely(x) likely(static_key_enabled(&(x)->key))
#define static_branch_unlikely(x) unlikely(static_key_enabled(&(x)->key))
#endif /* HAVE_JUMP_LABEL */
/*
* Advanced usage; refcount, branch is enabled when: count != 0
*/
#define static_branch_inc(x) static_key_slow_inc(&(x)->key)
#define static_branch_dec(x) static_key_slow_dec(&(x)->key)
/*
* Normal usage; boolean enable/disable.
*/
#define static_branch_enable(x) static_key_enable(&(x)->key)
#define static_branch_disable(x) static_key_disable(&(x)->key)
#endif /* _LINUX_JUMP_LABEL_H */
#endif /* __ASSEMBLY__ */