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SubjectThe SMP alternatives code breaks exception fixup?
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.



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