btrfs: remove no longer needed use of log_writers for the log root tree

When syncing the log, we used to update the log root tree without holding
neither the log_mutex of the subvolume root nor the log_mutex of log root
tree.

We used to have two critical sections delimited by the log_mutex of the
log root tree, so in the first one we incremented the log_writers of the
log root tree and on the second one we decremented it and waited for the
log_writers counter to go down to zero. This was because the update of
the log root tree happened between the two critical sections.

The use of two critical sections allowed a little bit more of parallelism
and required the use of the log_writers counter, necessary to make sure
we didn't miss any log root tree update when we have multiple tasks trying
to sync the log in parallel.

However after commit 06989c799f0481 ("Btrfs: fix race updating log root
item during fsync") the log root tree update was moved into a critical
section delimited by the subvolume's log_mutex. Later another commit
moved the log tree update from that critical section into the second
critical section delimited by the log_mutex of the log root tree. Both
commits addressed different bugs.

The end result is that the first critical section delimited by the
log_mutex of the log root tree became pointless, since there's nothing
done between it and the second critical section, we just have an unlock
of the log_mutex followed by a lock operation. This means we can merge
both critical sections, as the first one does almost nothing now, and we
can stop using the log_writers counter of the log root tree, which was
incremented in the first critical section and decremented in the second
criticial section, used to make sure no one in the second critical section
started writeback of the log root tree before some other task updated it.

So just remove the mutex_unlock() followed by mutex_lock() of the log root
tree, as well as the use of the log_writers counter for the log root tree.

This patch is part of a series that has the following patches:

1/4 btrfs: only commit the delayed inode when doing a full fsync
2/4 btrfs: only commit delayed items at fsync if we are logging a directory
3/4 btrfs: stop incremening log_batch for the log root tree when syncing log
4/4 btrfs: remove no longer needed use of log_writers for the log root tree

After the entire patchset applied I saw about 12% decrease on max latency
reported by dbench. The test was done on a qemu vm, with 8 cores, 16Gb of
ram, using kvm and using a raw NVMe device directly (no intermediary fs on
the host). The test was invoked like the following:

  mkfs.btrfs -f /dev/sdk
  mount -o ssd -o nospace_cache /dev/sdk /mnt/sdk
  dbench -D /mnt/sdk -t 300 8
  umount /mnt/dsk

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

2 files changed, 1 insertions, 13 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 726e2de685ce..c083628e96e8 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1060,6 +1060,7 @@ struct btrfs_root {
 	wait_queue_head_t log_writer_wait;
 	wait_queue_head_t log_commit_wait[2];
 	struct list_head log_ctxs[2];
+	/* Used only for log trees of subvolumes, not for the log root tree */
 	atomic_t log_writers;
 	atomic_t log_commit[2];
 	/* Used only for log trees of subvolumes, not for the log root tree */
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index ccc6b59f6729..aaa449153d9c 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3116,28 +3116,17 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 	btrfs_init_log_ctx(&root_log_ctx, NULL);
 
 	mutex_lock(&log_root_tree->log_mutex);
-	atomic_inc(&log_root_tree->log_writers);
 
 	index2 = log_root_tree->log_transid % 2;
 	list_add_tail(&root_log_ctx.list, &log_root_tree->log_ctxs[index2]);
 	root_log_ctx.log_transid = log_root_tree->log_transid;
 
-	mutex_unlock(&log_root_tree->log_mutex);
-
-	mutex_lock(&log_root_tree->log_mutex);
-
 	/*
 	 * Now we are safe to update the log_root_tree because we're under the
 	 * log_mutex, and we're a current writer so we're holding the commit
 	 * open until we drop the log_mutex.
 	 */
 	ret = update_log_root(trans, log, &new_root_item);
-
-	if (atomic_dec_and_test(&log_root_tree->log_writers)) {
-		/* atomic_dec_and_test implies a barrier */
-		cond_wake_up_nomb(&log_root_tree->log_writer_wait);
-	}
-
 	if (ret) {
 		if (!list_empty(&root_log_ctx.list))
 			list_del_init(&root_log_ctx.list);
@@ -3183,8 +3172,6 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
 				root_log_ctx.log_transid - 1);
 	}
 
-	wait_for_writer(log_root_tree);
-
 	/*
 	 * now that we've moved on to the tree of log tree roots,
 	 * check the full commit flag again