[tip:locking/urgent] locking/rwsem: Add stack frame dependency for ____down_write()

[tip-bot for Josh Poimboeuf]

Commit-ID:  9a0f9948b35140ebb8aa9b753e3ed09975700483
Gitweb:     http://git.kernel.org/tip/9a0f9948b35140ebb8aa9b753e3ed09975700483
Author:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Thu, 13 Oct 2016 16:26:15 -0500
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sun, 16 Oct 2016 11:17:58 +0200

locking/rwsem: Add stack frame dependency for ____down_write()

Arnd reported the following objtool warning:

  kernel/locking/rwsem.o: warning: objtool: down_write_killable()+0x16: call without frame pointer save/setup

The warning means gcc placed the ____down_write() inline asm (and its
call instruction) before the frame pointer setup in
down_write_killable(), which breaks frame pointer convention and can
result in incorrect stack traces.

Force the stack frame to be created before the call instruction by
listing the stack pointer as an output operand in the inline asm
statement.

Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1188b7015f04baf361e59de499ee2d7272c59dce.1476393828.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/rwsem.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index 3d33a71..a34e0d4 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -103,8 +103,10 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
 ({							\
 	long tmp;					\
 	struct rw_semaphore* ret;			\
+	register void *__sp asm(_ASM_SP);		\
+							\
 	asm volatile("# beginning down_write\n\t"	\
-		     LOCK_PREFIX "  xadd      %1,(%3)\n\t"	\
+		     LOCK_PREFIX "  xadd      %1,(%4)\n\t"	\
 		     /* adds 0xffff0001, returns the old value */ \
 		     "  test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" \
 		     /* was the active mask 0 before? */\
@@ -112,7 +114,7 @@ static inline bool __down_read_trylock(struct rw_semaphore *sem)
 		     "  call " slow_path "\n"		\
 		     "1:\n"				\
 		     "# ending down_write"		\
-		     : "+m" (sem->count), "=d" (tmp), "=a" (ret)	\
+		     : "+m" (sem->count), "=d" (tmp), "=a" (ret), "+r" (__sp) \
 		     : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) \
 		     : "memory", "cc");			\
 	ret;						\