fix block device size update serialization

From: Christoph Hellwig
Date: Fri Aug 21 2020 - 04:57:13 EST


Hi Jens,

this series fixes how we update i_size for the block device inodes (and
thus the block device). Different helpers use two different locks
(bd_mutex and i_rwsem) to protect the update, and it appears device
mapper uses yet another internal lock. A lot of the drivers do the
update handcrafted in often crufty ways. And in addition to that mess
it turns out that the "main" lock, bd_mutex is pretty dead lock prone
vs other spots in the block layer that acquire it during revalidation
operations, as reported by Xianting.

Fix all that by adding a dedicated spinlock just for the size updates.