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    SubjectRe: [PATCH] drm: Fix lock order reversal between mmap_sem and struct_mutex.
    On Thu, 2009-02-19 at 22:02 +0100, Thomas Hellstrom wrote:
    > It looks to me like the driver preferred locking order is
    > object_mutex (which happens to be the device global struct_mutex)
    > mmap_sem
    > offset_mutex.
    > So if one could avoid using the struct_mutex for object bookkeeping (A
    > separate lock) then
    > vm_open() and vm_close() would adhere to that locking order as well,
    > simply by not taking the struct_mutex at all.
    > So only fault() remains, in which that locking order is reversed.
    > Personally I think the trylock ->reschedule->retry method with proper
    > commenting is a good solution. It will be the _only_ place where locking
    > order is reversed and it is done in a deadlock-safe manner. Note that
    > fault() doesn't really fail, but requests a retry from user-space with
    > rescheduling to give the process holding the struct_mutex time to
    > release it.

    It doesn't do the reschedule -- need_resched() will check if the current
    task was marked to be scheduled away, furthermore yield based locking
    sucks chunks.

    What's so very difficult about pulling the copy_*_user() out from under
    the locks?

     \ /
      Last update: 2009-02-19 23:29    [W:0.020 / U:8.432 seconds]
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