Messages in this thread | | | Date | Fri, 27 Mar 2009 06:50:53 +0100 | From | Eric Dumazet <> | Subject | Re: Question about PRIVATE_FUTEX |
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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.
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...
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