kill-the-bkl/reiserfs: reduce number of contentions in search_by_key()

search_by_key() is a central function in reiserfs which searches
the patch in the fs tree from the root to a node given its key.

It is the function that is most requesting the write lock
because it's a path very often used.

Also we forget to release the lock while reading the next tree node,
making us holding the lock in a wasteful way.

Then we release the lock while reading the current node and its childs,
all-in-one. It should be safe because we have a reference to these
blocks and even if we read a block that will be concurrently changed,
we have an fs_changed check later that will make us retry the path from
the root.

[ Impact: release the write lock while unused in a hot path ]

Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>

fs/reiserfs/stree.c

@@ -529,6 +529,14 @@ static void search_by_key_reada(struct super_block *s,
 	for (i = 0; i < num; i++) {
 		bh[i] = sb_getblk(s, b[i]);
 	}
+	/*
+	 * We are going to read some blocks on which we
+	 * have a reference. It's safe, though we might be
+	 * reading blocks concurrently changed if we release
+	 * the lock. But it's still fine because we check later
+	 * if the tree changed
+	 */
+	reiserfs_write_unlock(s);
 	for (j = 0; j < i; j++) {
 		/*
 		 * note, this needs attention if we are getting rid of the BKL
@@ -626,10 +634,12 @@ int search_by_key(struct super_block *sb, const struct cpu_key *key,	/* Key to s
 		if ((bh = last_element->pe_buffer =
 		     sb_getblk(sb, block_number))) {
 			if (!buffer_uptodate(bh) && reada_count > 1)
+				/* will unlock the write lock */
 				search_by_key_reada(sb, reada_bh,
 						    reada_blocks, reada_count);
-			ll_rw_block(READ, 1, &bh);
+			else
 				reiserfs_write_unlock(sb);
+			ll_rw_block(READ, 1, &bh);
 			wait_on_buffer(bh);
 			reiserfs_write_lock(sb);
 			if (!buffer_uptodate(bh))