[PATCH v2 2/5] btrfs: Add a lockdep model for the num_extwriters wait event

[Ioannis Angelakopoulos <iangelak@fb.com>]

Similarly to the num_writers wait event in fs/btrfs/transaction.c add a
lockdep annotation for the num_extwriters wait event.

Use a read/write lockdep map for the annotation. A thread starting/joining
the transaction acquires the map as a reader when it increments
cur_trans->num_writers and it acquires the map as a writer before it
blocks on the wait event.

Signed-off-by: Ioannis Angelakopoulos <iangelak@fb.com>
---
 fs/btrfs/ctree.h       |  1 +
 fs/btrfs/disk-io.c     |  4 ++++
 fs/btrfs/transaction.c | 13 +++++++++++++
 3 files changed, 18 insertions(+)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 999868734be7..586756f831e5 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1096,6 +1096,7 @@ struct btrfs_fs_info {
 	struct btrfs_commit_stats commit_stats;
 
 	struct lockdep_map btrfs_trans_num_writers_map;
+	struct lockdep_map btrfs_trans_num_extwriters_map;
 
 #ifdef CONFIG_BTRFS_FS_REF_VERIFY
 	spinlock_t ref_verify_lock;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 01a5a49a3a11..b1193584ba49 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3047,6 +3047,7 @@ static int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
 void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
 {
 	static struct lock_class_key btrfs_trans_num_writers_key;
+	static struct lock_class_key btrfs_trans_num_extwriters_key;
 
 	INIT_RADIX_TREE(&fs_info->fs_roots_radix, GFP_ATOMIC);
 	INIT_RADIX_TREE(&fs_info->buffer_radix, GFP_ATOMIC);
@@ -3079,6 +3080,9 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
 	lockdep_init_map(&fs_info->btrfs_trans_num_writers_map,
 					 "btrfs_trans_num_writers",
 					 &btrfs_trans_num_writers_key, 0);
+	lockdep_init_map(&fs_info->btrfs_trans_num_extwriters_map,
+					 "btrfs_trans_num_extwriters",
+					 &btrfs_trans_num_extwriters_key, 0);
 
 	INIT_LIST_HEAD(&fs_info->dirty_cowonly_roots);
 	INIT_LIST_HEAD(&fs_info->space_info);
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index d8287ec890bc..c9751a05c029 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -314,6 +314,7 @@ static noinline int join_transaction(struct btrfs_fs_info *fs_info,
 		extwriter_counter_inc(cur_trans, type);
 		spin_unlock(&fs_info->trans_lock);
 		btrfs_lockdep_acquire(fs_info, btrfs_trans_num_writers);
+		btrfs_lockdep_acquire(fs_info, btrfs_trans_num_extwriters);
 		return 0;
 	}
 	spin_unlock(&fs_info->trans_lock);
@@ -336,6 +337,7 @@ static noinline int join_transaction(struct btrfs_fs_info *fs_info,
 		return -ENOMEM;
 
 	btrfs_lockdep_acquire(fs_info, btrfs_trans_num_writers);
+	btrfs_lockdep_acquire(fs_info, btrfs_trans_num_extwriters);
 
 	spin_lock(&fs_info->trans_lock);
 	if (fs_info->running_transaction) {
@@ -343,11 +345,13 @@ static noinline int join_transaction(struct btrfs_fs_info *fs_info,
 		 * someone started a transaction after we unlocked.  Make sure
 		 * to redo the checks above
 		 */
+		btrfs_lockdep_release(fs_info, btrfs_trans_num_extwriters);
 		btrfs_lockdep_release(fs_info, btrfs_trans_num_writers);
 		kfree(cur_trans);
 		goto loop;
 	} else if (BTRFS_FS_ERROR(fs_info)) {
 		spin_unlock(&fs_info->trans_lock);
+		btrfs_lockdep_release(fs_info, btrfs_trans_num_extwriters);
 		btrfs_lockdep_release(fs_info, btrfs_trans_num_writers);
 		kfree(cur_trans);
 		return -EROFS;
@@ -1028,6 +1032,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 
 	cond_wake_up(&cur_trans->writer_wait);
 
+	btrfs_lockdep_release(info, btrfs_trans_num_extwriters);
 	btrfs_lockdep_release(info, btrfs_trans_num_writers);
 
 	btrfs_put_transaction(cur_trans);
@@ -2270,6 +2275,13 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 	if (ret)
 		goto lockdep_release;
 
+	/*
+	 * The thread has started/joined the transaction thus it holds the lockdep
+	 * map as a reader. It has to release it before acquiring the lockdep map
+	 * as a writer.
+	 */
+	btrfs_lockdep_release(fs_info, btrfs_trans_num_extwriters);
+	btrfs_might_wait_for_event(fs_info, btrfs_trans_num_extwriters);
 	wait_event(cur_trans->writer_wait,
 		   extwriter_counter_read(cur_trans) == 0);
 
@@ -2540,6 +2552,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans)
 
 	return ret;
 lockdep_release:
+	btrfs_lockdep_release(fs_info, btrfs_trans_num_extwriters);
 	btrfs_lockdep_release(fs_info, btrfs_trans_num_writers);
 	goto cleanup_transaction;
 }
-- 
2.30.2