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SubjectRe: The SMP alternatives code breaks exception fixup?
On 01/21/2008 03:47 PM, Chuck Ebbert wrote:
> Looking at the oops report from this bug:
>
> [bugzilla] https://bugzilla.redhat.com/show_bug.cgi?id=429412
> [oops] https://bugzilla.redhat.com/attachment.cgi?id=292260
>
> It was an unhandled page fault (protection violation.)
>
> I tracked it down to the cmpxchg in this code:
>
> include/asm-x86/futex_32.h::futex_atomic_cmpxchg_inatomic()
> __asm__ __volatile__(
> "1: " LOCK_PREFIX "cmpxchgl %3, %1 \n"
> "2: .section .fixup, \"ax\" \n"
> "3: mov %2, %0 \n"
> " jmp 2b \n"
> " .previous \n"
> " .section __ex_table, \"a\" \n"
> " .align 8 \n"
> " .long 1b,3b \n"
> " .previous \n"
>
> 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.
>

The bug reporter has confirmed that booting with "noreplace-smp" prevents
the kernel oops.


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