Re: Question about PRIVATE_FUTEX

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Mar 27, 2009 at 2:50 PM, Eric Dumazet <dada1@cosmosbay.com> wrote:
> Minchan Kim a écrit :
>> Thanks for kind explanation.
>>
>> On Fri, Mar 27, 2009 at 1:56 PM, Eric Dumazet <dada1@cosmosbay.com> wrote:
>>> Minchan Kim a écrit :
>>>> Hi, Peter and Eric.
>>>>
>>>> I am not expert about futex.
>>>> I am sorry if this is dumb question.
>>>>
>>>> If we use private futex, get_futex_key don't call get_user_pages_fast
>>>> which pins page at page table.
>>>> 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 ?
>>>>
>>> Nothing makes sure user page is mapped, as we dont have to (for private futexes
>>> at least, since the 'key' is a combination of the futex virtual address (not
>>> depending on the underlying physical page) and the task mm (sort of a static
>>> offset per task)
>>> If no page is mapped, a normal error should be returned to user, since
>>> access to futex location will trigger a fault.
>>>
>>
>> I mean as follows.
>> It seems even shared futex case.
>>
>> After calling get_user_pages_fast, get_futex_key calls unlock_page and
>> put_page, too.  Then futex_wait  calls get_futex_value_locked.
>>
>> Generally, current page->count is one and nolocked.
>> I think kernel reclaimer can reclaim the page.
>>
>> Wouldn't kernel reclaim the page between get_fuex_key and
>> get_futex_value_locked ?
>> If kernel reclaimed the page, __copy_from_user_inatomic can happens
>> page fault although pagefault_disable is on.
>>
>> How do we make sure this race condition ?
>> Do I miss something ?
>>
>
> Hmmm, so your question is not about PRIVATE futexes, but shared ones.
>
> I guess if page is no more present, its not a problem since
> get_futex_value_locked() returns an error. We then take a slow
> path, calling get_user() and retrying whole futex logic.

Indeed.
I misunderstood about __copy_from_user_inatomic.
It never sleep.

> However, comment at line 1213 is misleading I guess, since
> we dont hold mmap semaphore anymore ?
>
>         * for shared futexes, we hold the mmap semaphore, so the mapping
>         * cannot have changed since we looked it up in get_futex_key.
>         */
>        ret = get_futex_value_locked(&uval, uaddr);
>
> So if page was un-mapped by another thread, and re-mapped to another physical
> page, then this thread might sleep on 'kernel futex' not anymore reachable...
>
> User error, as it is not supposed to happen in a sane program, undefined
> result...

Yes. How about removing confusing comments ?

Thanks for great explanation :)

-- 
Kinds regards,
Minchan Kim