Re: [stable] The SMP alternatives code breaks exception fixup?

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> > I don't see this in Linus's tree, am I just missing it?  Do you have 
> > a git commit id?
> 
> Isn't this 9d55b9923a1b7ea8193b8875c57ec940dc2ff027 (possibly with 
> 2532ec6d178abc55681d049097d3dc577eaa266c on top)?

yeah.

Greg: note that this is against post-unification asm-x86/futex.h so it 
wont apply to .24.

I've backported the fix to .24 - find it below. (untested but obvious)

	Ingo

----------------------->
Subject: x86: replace LOCK_PREFIX in futex.h
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

The exception fixup for the futex macros __futex_atomic_op1/2 and
futex_atomic_cmpxchg_inatomic() is missing an entry when the lock
prefix is replaced by a NOP via SMP alternatives.

Chuck Ebert tracked this down from the information provided in:
https://bugzilla.redhat.com/show_bug.cgi?id=429412

A possible solution would be to add another fixup after the
LOCK_PREFIX, so both the LOCK and NOP case have their own entry in the
exception table, but it's not really worth the trouble.

Simply replace LOCK_PREFIX with lock and keep those untouched by SMP
alternatives.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 include/asm-x86/futex_32.h |    6 +++---
 include/asm-x86/futex_64.h |    6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

Index: linux-2.6.24/include/asm-x86/futex_32.h
===================================================================
--- linux-2.6.24.orig/include/asm-x86/futex_32.h
+++ linux-2.6.24/include/asm-x86/futex_32.h
@@ -28,7 +28,7 @@
 "1:	movl	%2, %0\n\
 	movl	%0, %3\n"					\
 	insn "\n"						\
-"2:	" LOCK_PREFIX "cmpxchgl %3, %2\n\
+"2:	lock; cmpxchgl %3, %2\n					\
 	jnz	1b\n\
 3:	.section .fixup,\"ax\"\n\
 4:	mov	%5, %1\n\
@@ -68,7 +68,7 @@ futex_atomic_op_inuser (int encoded_op, 
 #endif
 		switch (op) {
 		case FUTEX_OP_ADD:
-			__futex_atomic_op1(LOCK_PREFIX "xaddl %0, %2", ret,
+			__futex_atomic_op1("lock; xaddl %0, %2", ret, oldval,
 					   oldval, uaddr, oparg);
 			break;
 		case FUTEX_OP_OR:
@@ -111,8 +111,8 @@ futex_atomic_cmpxchg_inatomic(int __user
 		return -EFAULT;
 
 	__asm__ __volatile__(
-		"1:	" LOCK_PREFIX "cmpxchgl %3, %1		\n"
 
+		"1:	lock; cmpxchgl %3, %1			\n"
 		"2:	.section .fixup, \"ax\"			\n"
 		"3:	mov     %2, %0				\n"
 		"	jmp     2b				\n"
Index: linux-2.6.24/include/asm-x86/futex_64.h
===================================================================
--- linux-2.6.24.orig/include/asm-x86/futex_64.h
+++ linux-2.6.24/include/asm-x86/futex_64.h
@@ -27,7 +27,7 @@
 "1:	movl	%2, %0\n\
 	movl	%0, %3\n"					\
 	insn "\n"						\
-"2:	" LOCK_PREFIX "cmpxchgl %3, %2\n\
+"2:	lock; cmpxchgl %3, %2\n					\
 	jnz	1b\n\
 3:	.section .fixup,\"ax\"\n\
 4:	mov	%5, %1\n\
@@ -62,7 +62,7 @@ futex_atomic_op_inuser (int encoded_op, 
 		__futex_atomic_op1("xchgl %0, %2", ret, oldval, uaddr, oparg);
 		break;
 	case FUTEX_OP_ADD:
-		__futex_atomic_op1(LOCK_PREFIX "xaddl %0, %2", ret, oldval,
+		__futex_atomic_op1("lock; xaddl %0, %2", ret, oldval,
 				   uaddr, oparg);
 		break;
 	case FUTEX_OP_OR:
@@ -101,8 +101,8 @@ futex_atomic_cmpxchg_inatomic(int __user
 		return -EFAULT;
 
 	__asm__ __volatile__(
-		"1:	" LOCK_PREFIX "cmpxchgl %3, %1		\n"
 
+		"1:	lock; cmpxchgl %3, %1			\n"
 		"2:	.section .fixup, \"ax\"			\n"
 		"3:	mov     %2, %0				\n"
 		"	jmp     2b				\n"
--
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