locking/static_keys: Add a new static_key interface
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:
a833581e372a ("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>
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 2e7cc1e..8fd00d8 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -165,16 +165,22 @@
static inline struct static_key *jump_entry_key(struct jump_entry *entry)
{
- return (struct static_key *)((unsigned long)entry->key);
+ return (struct static_key *)((unsigned long)entry->key & ~1UL);
+}
+
+static bool jump_entry_branch(struct jump_entry *entry)
+{
+ return (unsigned long)entry->key & 1UL;
}
static enum jump_label_type jump_label_type(struct jump_entry *entry)
{
struct static_key *key = jump_entry_key(entry);
bool enabled = static_key_enabled(key);
- bool type = static_key_type(key);
+ bool branch = jump_entry_branch(entry);
- return enabled ^ type;
+ /* See the comment in linux/jump_label.h */
+ return enabled ^ branch;
}
static void __jump_label_update(struct static_key *key,
@@ -205,7 +211,10 @@
for (iter = iter_start; iter < iter_stop; iter++) {
struct static_key *iterk;
- arch_jump_label_transform_static(iter, jump_label_type(iter));
+ /* rewrite NOPs */
+ if (jump_label_type(iter) == JUMP_LABEL_NOP)
+ arch_jump_label_transform_static(iter, JUMP_LABEL_NOP);
+
iterk = jump_entry_key(iter);
if (iterk == key)
continue;
@@ -225,6 +234,16 @@
#ifdef CONFIG_MODULES
+static enum jump_label_type jump_label_init_type(struct jump_entry *entry)
+{
+ struct static_key *key = jump_entry_key(entry);
+ bool type = static_key_type(key);
+ bool branch = jump_entry_branch(entry);
+
+ /* See the comment in linux/jump_label.h */
+ return type ^ branch;
+}
+
struct static_key_mod {
struct static_key_mod *next;
struct jump_entry *entries;
@@ -276,8 +295,11 @@
if (iter_start == iter_stop)
return;
- for (iter = iter_start; iter < iter_stop; iter++)
- arch_jump_label_transform_static(iter, JUMP_LABEL_NOP);
+ for (iter = iter_start; iter < iter_stop; iter++) {
+ /* Only write NOPs for arch_branch_static(). */
+ if (jump_label_init_type(iter) == JUMP_LABEL_NOP)
+ arch_jump_label_transform_static(iter, JUMP_LABEL_NOP);
+ }
}
static int jump_label_add_module(struct module *mod)
@@ -318,7 +340,8 @@
jlm->next = key->next;
key->next = jlm;
- if (jump_label_type(iter) == JUMP_LABEL_JMP)
+ /* Only update if we've changed from our initial state */
+ if (jump_label_type(iter) != jump_label_init_type(iter))
__jump_label_update(key, iter, iter_stop);
}
