On Tue, Dec 10, 2013 at 1:54 AM, Finn Thain <[email protected]> wrote:
> futex_init()
> cmpxchg_futex_value_locked(&curval, NULL, 0, 0)
> pagefault_disable();
> futex_atomic_cmpxchg_inatomic(curval, NULL, 0, 0)
> get_user(val, NULL)
>
> That is, futex_init() passes a NULL pointer expecting it to fault, as
> described in the comments in kernel/futex.c. Clearly the fault is not
> expected to be fatal.
I added some debugging (basically enabling DEBUG in
arch/m68k/kernel/traps.c, but dependent on a flag that's set to 1 in
futex_init(), and a check in do_page_fault()), and ran it on ARAnyM,
to get a grasp of what's happening on '040:
futex_atomic_cmpxchg_inatomic:20
*** Bus Error *** Format is 7
ssw=0x505, fa=0x0
wb1s=0x0, wb2s=0x0, wb3s=0x0
wb2a=0, wb3a=0, wb2d=0, wb3d=0
mmusr = 800
in_atomic() = 1
mm = (null)
no_context
send_fault_sig: (null),7,0
do_page_fault() !=0
.. disabling wb2
Summarized: access_error040() calls do_page_fault(), which just sends
SIGBUS (7), as in_atomic() is 1.
> Data read fault at 0x00000000 in Super Data (pc=0x3afec)
> BAD KERNEL BUSERR
On '030, the relevant code is:
if (mmusr & (MMU_I | MMU_WP)) {
if (ssw & 4) {
printk("Data %s fault at %#010lx in %s (pc=%#lx)\n",
ssw & RW ? "read" : "write",
fp->un.fmtb.daddr,
space_names[ssw & DFC], fp->ptregs.pc);
goto buserr;
}
/* Don't try to do anything further if an exception was
handled. */
if (do_page_fault (&fp->ptregs, addr, errorcode) < 0)
return;
But we never get to do_page_fault(), as ssw = 5 (SUPER_DATA).
The "if (ssw & 4) { ... }" chunk was added in commit
e48d483d581278fae02a5fffeba2b1fef47be4d4 (from full-history-linux):
Author: Andrew Morton <[email protected]>
Date: Sun Jan 18 18:32:30 2004 -0800
[PATCH] M68k RMW accesses
From: Geert Uytterhoeven <[email protected]>
M68k: Avoid bus fault for certain RMW accesses (from Roman Zippel)
Which originates from a CVS commit in 2003, based on v2 (v1 didn't have
the chunk) of a patch in the thread "RMW instructions on
MC68020/MC68851 combo..." between Kars and Roman Z.
I guess this case will work(TM) if you remove that chunk again?
But what are the other implications of that?
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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