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>

1 files changed, 17 insertions, 8 deletions

diff --git a/arch/arm/include/asm/jump_label.h b/arch/arm/include/asm/jump_label.h
index 5f337dc5c108..34f7b6980d21 100644
--- a/arch/arm/include/asm/jump_label.h
+++ b/arch/arm/include/asm/jump_label.h
@@ -4,23 +4,32 @@
 #ifndef __ASSEMBLY__
 
 #include <linux/types.h>
+#include <asm/unified.h>
 
 #define JUMP_LABEL_NOP_SIZE 4
 
-#ifdef CONFIG_THUMB2_KERNEL
-#define JUMP_LABEL_NOP	"nop.w"
-#else
-#define JUMP_LABEL_NOP	"nop"
-#endif
+static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+{
+	asm_volatile_goto("1:\n\t"
+		 WASM(nop) "\n\t"
+		 ".pushsection __jump_table,  \"aw\"\n\t"
+		 ".word 1b, %l[l_yes], %c0\n\t"
+		 ".popsection\n\t"
+		 : :  "i" (&((char *)key)[branch]) :  : l_yes);
+
+	return false;
+l_yes:
+	return true;
+}
 
-static __always_inline bool arch_static_branch(struct static_key *key)
+static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
 {
 	asm_volatile_goto("1:\n\t"
-		 JUMP_LABEL_NOP "\n\t"
+		 WASM(b) " %l[l_yes]\n\t"
 		 ".pushsection __jump_table,  \"aw\"\n\t"
 		 ".word 1b, %l[l_yes], %c0\n\t"
 		 ".popsection\n\t"
-		 : :  "i" (key) :  : l_yes);
+		 : :  "i" (&((char *)key)[branch]) :  : l_yes);
 
 	return false;
 l_yes: