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From: Chuck Ebbert <cebbert@redhat.com>
To: linux-kernel <linux-kernel@vger.kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>, Gerd Hoffmann <kraxel@redhat.com>
Subject: The SMP alternatives code breaks exception fixup?
Date: Mon, 21 Jan 2008 15:47:50 -0500	[thread overview]
Message-ID: <479504F6.4080005@redhat.com> (raw)

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.


             reply	other threads:[~2008-01-21 20:48 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-21 20:47 Chuck Ebbert [this message]
2008-01-21 21:25 ` The SMP alternatives code breaks exception fixup? Chuck Ebbert
2008-01-22  5:26 ` Andi Kleen
2008-01-22 12:35   ` Thomas Gleixner
2008-01-22 20:19     ` Chuck Ebbert
2008-02-06 23:10       ` [stable] " Greg KH
2008-02-06 23:36         ` Linus Torvalds
2008-02-06 23:43           ` Ingo Molnar
2008-02-07  0:00             ` Greg KH
2008-02-07  6:55             ` Greg KH
2008-02-07 12:16         ` Thomas Gleixner

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