Messages in this thread |  | | Subject | Re: [PATCH] drm: Fix lock order reversal between mmap_sem and struct_mutex. | | From | Peter Zijlstra <> | | Date | Thu, 19 Feb 2009 23:26:12 +0100 |
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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?
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