compiler/gcc4: Add quirk for 'asm goto' miscompilation bug

commit 3f0116c3 upstream.

Fengguang Wu, Oleg Nesterov and Peter Zijlstra tracked down
a kernel crash to a GCC bug: GCC miscompiles certain 'asm goto'
constructs, as outlined here:

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670

Implement a workaround suggested by Jakub Jelinek.
Reported-and-tested-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[bwh: Backported to 3.2:
 - Drop inapplicable changes
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/mips/include/asm/jump_label.h

@@ -22,7 +22,7 @@
 
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
-	asm goto("1:\tnop\n\t"
+	asm_volatile_goto("1:\tnop\n\t"
 		"nop\n\t"
 		".pushsection __jump_table,  \"aw\"\n\t"
 		WORD_INSN " 1b, %l[l_yes], %0\n\t"

arch/powerpc/include/asm/jump_label.h

@@ -19,7 +19,7 @@
 
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
-	asm goto("1:\n\t"
+	asm_volatile_goto("1:\n\t"
 		 "nop\n\t"
 		 ".pushsection __jump_table,  \"aw\"\n\t"
 		 JUMP_ENTRY_TYPE "1b, %l[l_yes], %c0\n\t"

arch/s390/include/asm/jump_label.h

@@ -15,7 +15,7 @@
 
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
-	asm goto("0:	brcl 0,0\n"
+	asm_volatile_goto("0:	brcl 0,0\n"
 		".pushsection __jump_table, \"aw\"\n"
 		ASM_ALIGN "\n"
 		ASM_PTR " 0b, %l[label], %0\n"

arch/sparc/include/asm/jump_label.h

@@ -9,7 +9,7 @@
 
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
-		asm goto("1:\n\t"
+		asm_volatile_goto("1:\n\t"
 			 "nop\n\t"
 			 "nop\n\t"
 			 ".pushsection __jump_table,  \"aw\"\n\t"

arch/x86/include/asm/cpufeature.h

@@ -334,7 +334,7 @@ extern const char * const x86_power_flags[32];
 static __always_inline __pure bool __static_cpu_has(u16 bit)
 {
 #if __GNUC__ > 4 || __GNUC_MINOR__ >= 5
-		asm goto("1: jmp %l[t_no]\n"
+		asm_volatile_goto("1: jmp %l[t_no]\n"
 			 "2:\n"
 			 ".section .altinstructions,\"a\"\n"
 			 " .long 1b - .\n"

arch/x86/include/asm/jump_label.h

@@ -13,7 +13,7 @@
 
 static __always_inline bool arch_static_branch(struct jump_label_key *key)
 {
-	asm goto("1:"
+	asm_volatile_goto("1:"
 		JUMP_LABEL_INITIAL_NOP
 		".pushsection __jump_table,  \"aw\" \n\t"
 		_ASM_ALIGN "\n\t"

include/linux/compiler-gcc4.h

@@ -29,6 +29,21 @@
    the kernel context */
 #define __cold			__attribute__((__cold__))
 
+/*
+ * GCC 'asm goto' miscompiles certain code sequences:
+ *
+ *   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
+ *
+ * Work it around via a compiler barrier quirk suggested by Jakub Jelinek.
+ * Fixed in GCC 4.8.2 and later versions.
+ *
+ * (asm goto is automatically volatile - the naming reflects this.)
+ */
+#if GCC_VERSION <= 40801
+# define asm_volatile_goto(x...)	do { asm goto(x); asm (""); } while (0)
+#else
+# define asm_volatile_goto(x...)	do { asm goto(x); } while (0)
+#endif
 
 #if __GNUC_MINOR__ >= 5
 /*