Hi Heiko,
On Fri, Jan 3, 2014 at 3:19 PM, Heiko Carstens
<heiko.carstens@xxxxxxxxxx> wrote:
> your patch "futex: switch to USER_DS for futex test" in linux-next now causes
> s390 to die on boot
Oops, sorry for that. So Andrew was right to let it cook a bit longer in -next,
for 3.14.
> Now, this seems to be wrong. It was intended to cause a fault while in kernel
> space. When accessing user space current->mm must not be NULL, but it is, since
> this is early code and we have no user space context to operate with.
>
> Hence at least s390's page tables aren't setup yet to handle this correctly.
> Actually our code dies when walking page tables and trying to acquire current's
> mm->page_table_lock, which points to nowhere.
m68k do_page_fault() has:
/*
* If we're in an interrupt or have no user
* context, we must not take the fault..
*/
if (in_atomic() || !mm)
goto no_context;
so we abort if there's no mm (but in this specific case it was already
aborted due
to the in_atomic() test).
Actually s390 do_exception() has similar checks:
/*
* Verify that the fault happened in user space, that
* we are not in an interrupt and that there is a
* user context.
*/
fault = VM_FAULT_BADCONTEXT;
if (unlikely(!user_space_fault(trans_exc_code) || in_atomic() || !mm))
goto out;
But as it fails for you, it crashes before you get to that point?
> I'm wondering why m68k's exception handling for put/get_user doesn't fixup
> the instruction pointers and these functions simply return -EFAULT?
The fixup only happens for pointers in userspace context.
In kernel context, we die.
> Also m68k's futex_atomic_cmpxchg_inatomic() implementation seems to miss a
> page_fault_disable()/page_fault_enable() pair.
I'm not a futex hero, but FWIW, cmpxchg_futex_value_locked() calls
page_fault_disable() before calling futex_atomic_cmpxchg_inatomic().
The inatomic suffix indicates we're already in an atomic context, right?
> Since this is already the second or third time this specific futex code causes
> problems on s390, it would be nice if we could get rid of it. E.g. with the
> following patch:
Yeah, recently Linus was optimizing the code so the compiler would eliminate
the test for him...
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
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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