On Tue, 22 Jan 2008, Andi Kleen wrote:
> Chuck Ebbert <[EMAIL PROTECTED]> writes:
> >
> > There is a fixup, so this should never happen. But the lock instruction
> > was replaced with a nop by the altinstruction code, and that makes the fixup
> > address wrong. AFAICT we don't fix up the exception table when we replace
> > a lock with a nop, which makes the fixup table point to the nop instead
> > of the cmpxchg instruction and causes us to miss the fixup.
>
> Indeed. Nasty issue.
>
> A quick fix would be to add another fixup to handle both cases
Agreed.
> I checked the other LOCK_PREFIX users and they look ok.
Really ?
> Does this fix it?
No it does not. Chuck tracked it down to
include/asm-x86/futex_32.h::futex_atomic_cmpxchg_inatomic()
And your patch is changing: __futex_atomic_op2(). That's fine, because
we need the fixup for both places.
Also your patch is against x86.git and not against mainline. Corrected
version below. x86.git needs a different fixup once this hits
mainline, as it unifies the 32/64bit versions.
That's a long standing bug in both the PI futex and the standard futex
code. Needs to go to stable as well.
Thanks,
tglx
------------->
Subject: x86: fix missing exception entry for SMP alternatives in futex macros
From: Thomas Gleixner <[EMAIL PROTECTED]>
The exception fixup for the futex macros __futex_atomic_op2 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
The solution is to add another fixup after the LOCK_PREFIX, so both
the LOCK and NOP case have their own entry in the exception table.
The solution was pointed out by Andi Kleen.
Signed-off-by: Thomas Gleixner <[EMAIL PROTECTED]>
Acked-by: Ingo Molnar <[EMAIL PROTECTED]>
---
include/asm-x86/futex_32.h | 8 ++++----
include/asm-x86/futex_64.h | 8 ++++----
2 files changed, 8 insertions(+), 8 deletions(-)
Index: linux-2.6/include/asm-x86/futex_32.h
===================================================================
--- linux-2.6.orig/include/asm-x86/futex_32.h 2008-01-22 13:13:10.000000000
+0100
+++ linux-2.6/include/asm-x86/futex_32.h 2008-01-22 13:13:49.000000000
+0100
@@ -28,7 +28,7 @@
"1: movl %2, %0\n\
movl %0, %3\n" \
insn "\n" \
-"2: " LOCK_PREFIX "cmpxchgl %3, %2\n\
+"2: " LOCK_PREFIX "\n 5: cmpxchgl %3, %2\n\
jnz 1b\n\
3: .section .fixup,\"ax\"\n\
4: mov %5, %1\n\
@@ -36,7 +36,7 @@
.previous\n\
.section __ex_table,\"a\"\n\
.align 8\n\
- .long 1b,4b,2b,4b\n\
+ .long 1b,4b,2b,4b,5b,4b\n\
.previous" \
: "=&a" (oldval), "=&r" (ret), "+m" (*uaddr), \
"=&r" (tem) \
@@ -111,7 +111,7 @@ futex_atomic_cmpxchg_inatomic(int __user
return -EFAULT;
__asm__ __volatile__(
- "1: " LOCK_PREFIX "cmpxchgl %3, %1 \n"
+ "1: " LOCK_PREFIX "\n 4: cmpxchgl %3, %1 \n"
"2: .section .fixup, \"ax\" \n"
"3: mov %2, %0 \n"
@@ -120,7 +120,7 @@ futex_atomic_cmpxchg_inatomic(int __user
" .section __ex_table, \"a\" \n"
" .align 8 \n"
- " .long 1b,3b \n"
+ " .long 1b,3b,4b,3b \n"
" .previous \n"
: "=a" (oldval), "+m" (*uaddr)
Index: linux-2.6/include/asm-x86/futex_64.h
===================================================================
--- linux-2.6.orig/include/asm-x86/futex_64.h 2008-01-22 13:13:10.000000000
+0100
+++ linux-2.6/include/asm-x86/futex_64.h 2008-01-22 13:13:49.000000000
+0100
@@ -27,7 +27,7 @@
"1: movl %2, %0\n\
movl %0, %3\n" \
insn "\n" \
-"2: " LOCK_PREFIX "cmpxchgl %3, %2\n\
+"2: " LOCK_PREFIX "\n 5: cmpxchgl %3, %2\n\
jnz 1b\n\
3: .section .fixup,\"ax\"\n\
4: mov %5, %1\n\
@@ -35,7 +35,7 @@
.previous\n\
.section __ex_table,\"a\"\n\
.align 8\n\
- .quad 1b,4b,2b,4b\n\
+ .quad 1b,4b,2b,4b,5b,4b\n\
.previous" \
: "=&a" (oldval), "=&r" (ret), "=m" (*uaddr), \
"=&r" (tem) \
@@ -101,7 +101,7 @@ futex_atomic_cmpxchg_inatomic(int __user
return -EFAULT;
__asm__ __volatile__(
- "1: " LOCK_PREFIX "cmpxchgl %3, %1 \n"
+ "1: " LOCK_PREFIX "\n 4: cmpxchgl %3, %1 \n"
"2: .section .fixup, \"ax\" \n"
"3: mov %2, %0 \n"
@@ -110,7 +110,7 @@ futex_atomic_cmpxchg_inatomic(int __user
" .section __ex_table, \"a\" \n"
" .align 8 \n"
- " .quad 1b,3b \n"
+ " .quad 1b,3b,4b,3b \n"
" .previous \n"
: "=a" (oldval), "=m" (*uaddr)
--
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