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    SubjectRe: [PATCH, RFC] xip: use i_mutex for xip_file_fault

    Il 11/09/2011 13:25, Al Viro ha scritto:
    > On Sun, Sep 11, 2011 at 12:15:04PM +0100, Al Viro wrote:
    >> write() grabs ->i_mutex on the file it's going to write to. It uses
    >> copy_from_user() while holding ->i_mutex; that can end up calling ->fault().
    >> If your data comes from the same file mmapped in your address space, you
    >> have xip_write_fault() called while you are in xip_file_write() and *already*
    >> are holding ->i_mutex on the same inode. With your patch it will, AFAICS,
    >> cheerfully deadlock.
    > Oh, wait... You are only doing that to write side of pagefault? That's
    > better, but not much:
    > thread 1: mmap the file, modify mapping
    > thread 2: write() to file
    > The former will do xip_write_fault() while holding ->mmap_sem.
    > The latter will do copy_from_user() from xip_file_write(), getting
    > pagefaults while holding ->i_mutex.
    > Note that we are grabbing ->mmap_sem and ->i_mutex in opposite orders.
    > I.e. that will deadlock on you - all you need is threads sharing the
    > address space.

    Ok, thank you very much for the on-line debug :) So i_mutex is not a
    good lock to use in this situation. It was a common sync point, but it
    has some collateral effect on the write path that we must avoid. At this
    point, what can be a good strategy? Any opinion is welcome.


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