btrfs: simplify waiting loop in btrfs_tree_lock
Currently, the number of readers and writers is checked and in case
there are any, wait and redo the locks. There's some duplication
before the branches go back to again label, eg. calling wait_event on
blocking_readers twice.
The sequence is transformed
loop:
* wait for readers
* wait for writers
* write_lock
* check readers, unlock and wait for readers, loop
* check writers, unlock and wait for writers, loop
The new sequence is not exactly the same due to the simplification, for
readers it's slightly faster. For the writers, original code does
* wait for writers
* (loop) wait for readers
* wait for writers -- again
while the new goes directly to the reader check. This should behave the
same on a contended lock with multiple writers and readers, but can
reduce number of times we're waiting on something.
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>