Re: Question about PRIVATE_FUTEX

[Darren Hart <dvhltc@us.ibm.com>]

Minchan Kim wrote:
> On Fri, Mar 27, 2009 at 8:14 PM, Peter Zijlstra <peterz@infradead.org> 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