[PATCH RFC 3/9] btrfs: zoned: get rid of treelog_bg_lock

[Johannes Thumshirn <jth@kernel.org>]

From: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Lockstat analysis of benchmark workloads shows a very high contention of
the treelog_bg_lock. But the treelog_bg_lock only protects a single
field in 'struct btrfs_fs_info', namely 'u64 treelog_bg'.

Use READ_ONCE()/WRITE_ONCE() to access 'btrfs_fs_info::treelog_bg'.

This is safe in the allocator path, as treelog I/O is only going to block
groups in the treelog sub-space_info and at the moment, there is only one
treelog block group in this space info.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 fs/btrfs/disk-io.c     |  1 -
 fs/btrfs/extent-tree.c | 45 +++++++++++-------------------------------
 fs/btrfs/fs.h          |  1 -
 fs/btrfs/zoned.c       |  2 +-
 fs/btrfs/zoned.h       |  7 +++----
 5 files changed, 15 insertions(+), 41 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 9a13f5b1ed43..35cd38de7727 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2789,7 +2789,6 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
 	spin_lock_init(&fs_info->defrag_inodes_lock);
 	spin_lock_init(&fs_info->super_lock);
 	spin_lock_init(&fs_info->unused_bgs_lock);
-	spin_lock_init(&fs_info->treelog_bg_lock);
 	spin_lock_init(&fs_info->zone_active_bgs_lock);
 	rwlock_init(&fs_info->tree_mod_log_lock);
 	rwlock_init(&fs_info->global_root_lock);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index a9bda68a1883..46358a555f78 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3809,22 +3809,6 @@ static int do_allocation_clustered(struct btrfs_block_group *block_group,
 	return find_free_extent_unclustered(block_group, ffe_ctl);
 }
 
-/*
- * Tree-log block group locking
- * ============================
- *
- * fs_info::treelog_bg_lock protects the fs_info::treelog_bg which
- * indicates the starting address of a block group, which is reserved only
- * for tree-log metadata.
- *
- * Lock nesting
- * ============
- *
- * space_info::lock
- *   block_group::lock
- *     fs_info::treelog_bg_lock
- */
-
 /*
  * Simple allocator for sequential-only block group. It only allows sequential
  * allocation. No need to play with trees. This function also reserves the
@@ -3844,7 +3828,6 @@ static int do_allocation_zoned(struct btrfs_block_group *block_group,
 	u64 log_bytenr;
 	u64 data_reloc_bytenr;
 	int ret = 0;
-	bool skip = false;
 
 	ASSERT(btrfs_is_zoned(block_group->fs_info));
 
@@ -3852,13 +3835,9 @@ static int do_allocation_zoned(struct btrfs_block_group *block_group,
 	 * Do not allow non-tree-log blocks in the dedicated tree-log block
 	 * group, and vice versa.
 	 */
-	spin_lock(&fs_info->treelog_bg_lock);
-	log_bytenr = fs_info->treelog_bg;
+	log_bytenr = READ_ONCE(fs_info->treelog_bg);
 	if (log_bytenr && ((ffe_ctl->for_treelog && bytenr != log_bytenr) ||
 			   (!ffe_ctl->for_treelog && bytenr == log_bytenr)))
-		skip = true;
-	spin_unlock(&fs_info->treelog_bg_lock);
-	if (skip)
 		return 1;
 
 	/*
@@ -3894,14 +3873,13 @@ static int do_allocation_zoned(struct btrfs_block_group *block_group,
 
 	spin_lock(&space_info->lock);
 	spin_lock(&block_group->lock);
-	spin_lock(&fs_info->treelog_bg_lock);
 
 	if (ret)
 		goto out;
 
 	ASSERT(!ffe_ctl->for_treelog ||
-	       block_group->start == fs_info->treelog_bg ||
-	       fs_info->treelog_bg == 0);
+	       block_group->start == log_bytenr ||
+	       log_bytenr == 0);
 	ASSERT(!ffe_ctl->for_data_reloc ||
 	       block_group->start == data_reloc_bytenr ||
 	       data_reloc_bytenr == 0);
@@ -3917,7 +3895,7 @@ static int do_allocation_zoned(struct btrfs_block_group *block_group,
 	 * Do not allow currently using block group to be tree-log dedicated
 	 * block group.
 	 */
-	if (ffe_ctl->for_treelog && !fs_info->treelog_bg &&
+	if (ffe_ctl->for_treelog && log_bytenr == 0 &&
 	    (block_group->used || block_group->reserved)) {
 		ret = 1;
 		goto out;
@@ -3948,8 +3926,8 @@ static int do_allocation_zoned(struct btrfs_block_group *block_group,
 		goto out;
 	}
 
-	if (ffe_ctl->for_treelog && !fs_info->treelog_bg)
-		fs_info->treelog_bg = block_group->start;
+	if (ffe_ctl->for_treelog && READ_ONCE(fs_info->treelog_bg) == 0)
+		WRITE_ONCE(fs_info->treelog_bg, block_group->start);
 
 	if (ffe_ctl->for_data_reloc) {
 		if (READ_ONCE(fs_info->data_reloc_bg) == 0)
@@ -3987,10 +3965,9 @@ static int do_allocation_zoned(struct btrfs_block_group *block_group,
 
 out:
 	if (ret && ffe_ctl->for_treelog)
-		fs_info->treelog_bg = 0;
+		WRITE_ONCE(fs_info->treelog_bg, 0);
 	if (ret && ffe_ctl->for_data_reloc)
 		WRITE_ONCE(fs_info->data_reloc_bg, 0);
-	spin_unlock(&fs_info->treelog_bg_lock);
 	spin_unlock(&block_group->lock);
 	spin_unlock(&space_info->lock);
 	return ret;
@@ -4293,10 +4270,10 @@ static int prepare_allocation_zoned(struct btrfs_fs_info *fs_info,
 				    struct find_free_extent_ctl *ffe_ctl)
 {
 	if (ffe_ctl->for_treelog) {
-		spin_lock(&fs_info->treelog_bg_lock);
-		if (fs_info->treelog_bg)
-			ffe_ctl->hint_byte = fs_info->treelog_bg;
-		spin_unlock(&fs_info->treelog_bg_lock);
+		u64 treelog_bg = READ_ONCE(fs_info->treelog_bg);
+
+		if (treelog_bg)
+			ffe_ctl->hint_byte = treelog_bg;
 	} else if (ffe_ctl->for_data_reloc) {
 		u64 data_reloc_bg = READ_ONCE(fs_info->data_reloc_bg);
 
diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
index 570f4b85096c..a388af40a251 100644
--- a/fs/btrfs/fs.h
+++ b/fs/btrfs/fs.h
@@ -846,7 +846,6 @@ struct btrfs_fs_info {
 	u64 max_zone_append_size;
 
 	struct mutex zoned_meta_io_lock;
-	spinlock_t treelog_bg_lock;
 	u64 treelog_bg;
 
 	/* Start of the dedicated data relocation block group */
diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
index 388c277a84d3..c89f846af6dd 100644
--- a/fs/btrfs/zoned.c
+++ b/fs/btrfs/zoned.c
@@ -1948,7 +1948,7 @@ static bool check_bg_is_active(struct btrfs_eb_write_context *ctx,
 	if (test_bit(BLOCK_GROUP_FLAG_ZONE_IS_ACTIVE, &block_group->runtime_flags))
 		return true;
 
-	if (fs_info->treelog_bg == block_group->start) {
+	if (READ_ONCE(fs_info->treelog_bg) == block_group->start) {
 		if (!btrfs_zone_activate(block_group)) {
 			int ret_fin = btrfs_zone_finish_one_bg(fs_info);
 
diff --git a/fs/btrfs/zoned.h b/fs/btrfs/zoned.h
index 6e11533b8e14..c1b3a5c3a799 100644
--- a/fs/btrfs/zoned.h
+++ b/fs/btrfs/zoned.h
@@ -383,14 +383,13 @@ static inline void btrfs_zoned_meta_io_unlock(struct btrfs_fs_info *fs_info)
 static inline void btrfs_clear_treelog_bg(struct btrfs_block_group *bg)
 {
 	struct btrfs_fs_info *fs_info = bg->fs_info;
+	u64 treelog_bg = READ_ONCE(fs_info->treelog_bg);
 
 	if (!btrfs_is_zoned(fs_info))
 		return;
 
-	spin_lock(&fs_info->treelog_bg_lock);
-	if (fs_info->treelog_bg == bg->start)
-		fs_info->treelog_bg = 0;
-	spin_unlock(&fs_info->treelog_bg_lock);
+	if (treelog_bg == bg->start)
+		WRITE_ONCE(fs_info->treelog_bg, 0);
 }
 
 static inline void btrfs_zoned_data_reloc_lock(struct btrfs_inode *inode)
-- 
2.49.0