Re: Question about PRIVATE_FUTEX

[Darren Hart]

Minchan Kim wrote:
> On Fri, Mar 27, 2009 at 8:14 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Fri, 2009-03-27 at 19:56 +0900, Minchan Kim wrote:
> >
> > > > > Then, get_futex_value_locked calls __cpy_from_user_inatomic with
> > > > > pagefault_disable.
> > > > >
> > > > > Who make sure the user page is mapped at app's page table ?
> > > > Nobody, all uses of get_futex_value_locked() have to deal with it
> > > > returning -EFAULT.
> > > Does It mean that __copy_from_user_inatomic in get_futex_value_locked
> > > would be failed rather than sleep?
> > Correct.
> >
> > > In fact, I don't make sure _copy_from_user_inatomic function's meaning.
> > > As far as I understand, It never sleep. It just can be failed in case
> > > of user page isn't mapped. Is right ?
> > Correct.
> >
> > > Otherwise, it can be scheduled with pagefault_disable which increments
> > > preempt_count. It is a atomic bug.
> > > If my assume is right, it can be failed rather than sleep.
> > > At this case, other architecture implements __copy_from_user_inatomic
> > > with __copy_from_user which can be scheduled. It also can be bug.
> > >
> > > Hmm, Now I am confusing.
> > Confused I guess ;-)
> > The trick is in the in_atomic() check in the pagefault handler and the
> > fixup section of the copy routines.
>
> Whew~,  There was good hidden trick.
> I will dive into this assembly.
> I always thanks for your kindness. :)
>
> > #define __copy_user(to, from, size)                                     \
> > do {                                                                    \
> >        int __d0, __d1, __d2;                                           \
> >        __asm__ __volatile__(                                           \
> >                "       cmp  $7,%0\n"                                   \
> >                "       jbe  1f\n"                                      \
> >                "       movl %1,%0\n"                                   \
> >                "       negl %0\n"                                      \
> >                "       andl $7,%0\n"                                   \
> >                "       subl %0,%3\n"                                   \
> >                "4:     rep; movsb\n"                                   \
> >                "       movl %3,%0\n"                                   \
> >                "       shrl $2,%0\n"                                   \
> >                "       andl $3,%3\n"                                   \
> >                "       .align 2,0x90\n"                                \
> >                "0:     rep; movsl\n"                                   \
> >                "       movl %3,%0\n"                                   \
> >                "1:     rep; movsb\n"                                   \
> >                "2:\n"                                                  \
> >                ".section .fixup,\"ax\"\n"                              \
> >                "5:     addl %3,%0\n"                                   \
> >                "       jmp 2b\n"                                       \
> >                "3:     lea 0(%3,%0,4),%0\n"                            \
> >                "       jmp 2b\n"                                       \
> >                ".previous\n"                                           \
> >                ".section __ex_table,\"a\"\n"                           \
> >                "       .align 4\n"                                     \
> >                "       .long 4b,5b\n"                                  \
> >                "       .long 0b,3b\n"                                  \
> >                "       .long 1b,2b\n"                                  \
> >                ".previous"                                             \
> >                : "=&c"(size), "=&D" (__d0), "=&S" (__d1), "=r"(__d2)   \
> >                : "3"(size), "0"(size), "1"(to), "2"(from)              \
> >                : "memory");                                            \
> > } while (0)
> >
> > see that __ex_table section, it tells the fault handler where to
> > continue in case of an atomic fault.
> >
> > > > Most of this is legacy btw, from when futex ops were done under the
> > > > mmap_sem. Back then we couldn't fault because that would cause mmap_sem
> > > > recursion. Howver, now that we don't hold mmap_sem anymore we could use
> > > > a faulting user access like get_user().
> > > > Darren has been working on patches to clean that up, some of those are
> > > > already merged in the -tip tree.

I'm a little late to the party I guess.  Minchan, a lot of the fault 
logic has been cleaned up in the tip tree, core/futexes branch.  The 
removes a lot of the legacy complication from the faulting paths. 
However, the get_futex_key code remains the same if I remember correctly.

> > > Thanks for good information.
> > > It will be very desirable way to enhance kernel performance.
> > I doubt it'll make a measurable difference, if you need to fault
> > performance sucks anyway. If you don't, the current code is just as
> > fast.

Agreed.  If you are suffering performance hits from excessive paging, 
consider locking your memory.


--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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