A btree block cow has two parts, the first is to allocate a destination
block and the second is to copy the old bock over.
The first part needs locks in the extent allocation tree, and may need to
do IO. This changeset splits that into a separate function that can be
called without any tree locks held.
btrfs_search_slot is changed to drop its path and start over if it has
to COW a contended block. This often means that many writers will
pre-alloc a new destination for a the same contended block, but they
cache their prealloc for later use on lower levels in the tree.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
The allocation trees and the chunk trees are serialized via their own
dedicated mutexes. This means allocation location is still not very
fine grained.
The main FS btree is protected by locks on each block in the btree. Locks
are taken top / down, and as processing finishes on a given level of the
tree, the lock is released after locking the lower level.
The end result of a search is now a path where only the lowest level
is locked. Releasing or freeing the path drops any locks held.
Signed-off-by: Chris Mason <chris.mason@oracle.com>
