Btrfs: lock extents as we map them in DIO
A deadlock in xfstests 113 was uncovered by commit d187663e This is because we would not return EIOCBQUEUED for short AIO reads, instead we'd wait for the DIO to complete and then return the amount of data we transferred, which would allow our stuff to unlock the remaning amount. But with this change this no longer happens, so if we have a short AIO read (for example if we try to read past EOF), we could leave the section from EOF to the end of where we tried to read locked. Fixing this is tricky since there is no clear way to know exactly how much data DIO truly submitted for IO, so to make this less hard on ourselves and less combersome we need to lock the extents as we try to map them, and then we unlock any areas we didn't actually map. This makes us completely safe from deadlocks and reliance on a particular behavior of the DIO code. This also lays the groundwork for allowing us to use the normal csum storage method for reads which means we can remove an allocation. Thanks, Signed-off-by: Josef Bacik <jbacik@fusionio.com>
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