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>
diff --git a/fs/reiserfs/stree.c b/fs/reiserfs/stree.c
index 6ddcecb..960c911 100644
--- a/fs/reiserfs/stree.c
+++ b/fs/reiserfs/stree.c
@@ -529,6 +529,14 @@
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 @@
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);
+ else
+ reiserfs_write_unlock(sb);
ll_rw_block(READ, 1, &bh);
- reiserfs_write_unlock(sb);
wait_on_buffer(bh);
reiserfs_write_lock(sb);
if (!buffer_uptodate(bh))
