bcache: btree locking rework

Add a new lock, b->write_lock, which is required to actually modify - or write -
a btree node; this lock is only held for short durations.

This means we can write out a btree node without taking b->lock, which _is_ held
for long durations - solving a deadlock when btree_flush_write() (from the
journalling code) is called with a btree node locked.

Right now just occurs in bch_btree_set_root(), but with an upcoming journalling
rework is going to happen a lot more.

This also turns b->lock is now more of a read/intent lock instead of a
read/write lock - but not completely, since it still blocks readers. May turn it
into a real intent lock at some point in the future.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

1 files changed, 4 insertions, 5 deletions

diff --git a/drivers/md/bcache/journal.c b/drivers/md/bcache/journal.c
index c8bfc28cd2bd..59e82021b5bb 100644
--- a/drivers/md/bcache/journal.c
+++ b/drivers/md/bcache/journal.c
@@ -381,16 +381,15 @@ retry:
 
 	b = best;
 	if (b) {
-		rw_lock(true, b, b->level);
-
+		mutex_lock(&b->write_lock);
 		if (!btree_current_write(b)->journal) {
-			rw_unlock(true, b);
+			mutex_unlock(&b->write_lock);
 			/* We raced */
 			goto retry;
 		}
 
-		bch_btree_node_write(b, NULL);
-		rw_unlock(true, b);
+		__bch_btree_node_write(b, NULL);
+		mutex_unlock(&b->write_lock);
 	}
 }