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    SubjectRe: Question about PRIVATE_FUTEX
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    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
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