On Thu, 2009-02-19 at 18:04 -0800, Eric Anholt wrote: > On Thu, 2009-02-19 at 23:26 +0100, Peter Zijlstra wrote: > > 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.
Imagine what would happen if your faulting task was the highest RT prio task in the system, you'd end up with a life-lock. > > What's so very difficult about pulling the copy_*_user() out from under > > the locks? > > That we're expecting the data movement to occur while holding device > state in place. For example, we write data through the GTT most of the > time so we: > > lock struct_mutex > pin the object to the GTT > flushing caches as needed > copy_from_user > unpin object > unlock struct_mutex So you cannot drop the lock once you've pinned the dst object? > If I'm to pull the copy_from_user out, that means I have to: > > alloc temporary storage > for each block of temp storage size: > copy_from_user > lock struct_mutex > pin the object to the GTT > flush caches as needed > memcpy > unpin object > unlock struct_mutex > > At this point of introducing our third copy of the user's data in our > hottest path, we should probably ditch the pwrite path entirely and go > to user mapping of the objects for performance. Requiring user mapping > (which has significant overhead) cuts the likelihood of moving from > user-space object caching to kernel object caching in the future, which > has the potential of saving steaming piles of memory. Or you could get_user_pages() to fault the user pages and pin them, and then do pagefault_disable() and use copy_from_user_inatomic or such, and release the pages again. ------------------------------------------------------------------------------ Open Source Business Conference (OSBC), March 24-25, 2009, San Francisco, CA -OSBC tackles the biggest issue in open source: Open Sourcing the Enterprise -Strategies to boost innovation and cut costs with open source participation -Receive a $600 discount off the registration fee with the source code: SFAD http://p.sf.net/sfu/XcvMzF8H -- _______________________________________________ Dri-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/dri-devel
