bcachefs: snapshot_create_lock

Add a new lock for snapshot creation - this addresses a few races with
logged operations and snapshot deletion.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/bcachefs.h

@@ -746,6 +746,7 @@ struct bch_fs {
 	struct snapshot_table __rcu *snapshots;
 	size_t			snapshot_table_size;
 	struct mutex		snapshot_table_lock;
+	struct rw_semaphore	snapshot_create_lock;
 
 	struct work_struct	snapshot_delete_work;
 	struct work_struct	snapshot_wait_for_pagecache_and_delete_work;

fs/bcachefs/fs-ioctl.c

@@ -318,8 +318,8 @@ static int bch2_ioc_goingdown(struct bch_fs *c, u32 __user *arg)
 	return ret;
 }
 
-static long bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp,
-				struct bch_ioctl_subvolume arg)
+static long __bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp,
+					  struct bch_ioctl_subvolume arg)
 {
 	struct inode *dir;
 	struct bch_inode_info *inode;
@@ -440,6 +440,16 @@ static long bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp,
 	return error;
 }
 
+static long bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp,
+					struct bch_ioctl_subvolume arg)
+{
+	down_write(&c->snapshot_create_lock);
+	long ret = __bch2_ioctl_subvolume_create(c, filp, arg);
+	up_write(&c->snapshot_create_lock);
+
+	return ret;
+}
+
 static long bch2_ioctl_subvolume_destroy(struct bch_fs *c, struct file *filp,
 				struct bch_ioctl_subvolume arg)
 {

fs/bcachefs/io_misc.c

@@ -287,9 +287,18 @@ int bch2_truncate(struct bch_fs *c, subvol_inum inum, u64 new_i_size, u64 *i_sec
 	op.v.inum	= cpu_to_le64(inum.inum);
 	op.v.new_i_size	= cpu_to_le64(new_i_size);
 
-	return bch2_trans_run(c,
+	/*
+	 * Logged ops aren't atomic w.r.t. snapshot creation: creating a
+	 * snapshot while they're in progress, then crashing, will result in the
+	 * resume only proceeding in one of the snapshots
+	 */
+	down_read(&c->snapshot_create_lock);
+	int ret = bch2_trans_run(c,
 		bch2_logged_op_start(trans, &op.k_i) ?:
 		__bch2_resume_logged_op_truncate(trans, &op.k_i, i_sectors_delta));
+	up_read(&c->snapshot_create_lock);
+
+	return ret;
 }
 
 /* finsert/fcollapse: */
@@ -491,7 +500,16 @@ int bch2_fcollapse_finsert(struct bch_fs *c, subvol_inum inum,
 	op.v.src_offset	= cpu_to_le64(offset);
 	op.v.pos	= cpu_to_le64(insert ? U64_MAX : offset);
 
-	return bch2_trans_run(c,
+	/*
+	 * Logged ops aren't atomic w.r.t. snapshot creation: creating a
+	 * snapshot while they're in progress, then crashing, will result in the
+	 * resume only proceeding in one of the snapshots
+	 */
+	down_read(&c->snapshot_create_lock);
+	int ret = bch2_trans_run(c,
 		bch2_logged_op_start(trans, &op.k_i) ?:
 		__bch2_resume_logged_op_finsert(trans, &op.k_i, i_sectors_delta));
+	up_read(&c->snapshot_create_lock);
+
+	return ret;
 }

fs/bcachefs/snapshot.c

@@ -1447,6 +1447,8 @@ int bch2_delete_dead_snapshots(struct bch_fs *c)
 		}
 	}
 
+	down_write(&c->snapshot_create_lock);
+
 	for_each_btree_key(trans, iter, BTREE_ID_snapshots,
 			   POS_MIN, 0, k, ret) {
 		u32 snapshot = k.k->p.offset;
@@ -1457,6 +1459,9 @@ int bch2_delete_dead_snapshots(struct bch_fs *c)
 	}
 	bch2_trans_iter_exit(trans, &iter);
 
+	if (ret)
+		goto err_create_lock;
+
 	/*
 	 * Fixing children of deleted snapshots can't be done completely
 	 * atomically, if we crash between here and when we delete the interior
@@ -1467,14 +1472,14 @@ int bch2_delete_dead_snapshots(struct bch_fs *c)
 				  NULL, NULL, BTREE_INSERT_NOFAIL,
 		bch2_fix_child_of_deleted_snapshot(trans, &iter, k, &deleted_interior));
 	if (ret)
-		goto err;
+		goto err_create_lock;
 
 	darray_for_each(deleted, i) {
 		ret = commit_do(trans, NULL, NULL, 0,
 			bch2_snapshot_node_delete(trans, *i));
 		if (ret) {
 			bch_err_msg(c, ret, "deleting snapshot %u", *i);
-			goto err;
+			goto err_create_lock;
 		}
 	}
 
@@ -1483,11 +1488,13 @@ int bch2_delete_dead_snapshots(struct bch_fs *c)
 			bch2_snapshot_node_delete(trans, *i));
 		if (ret) {
 			bch_err_msg(c, ret, "deleting snapshot %u", *i);
-			goto err;
+			goto err_create_lock;
 		}
 	}
 
 	clear_bit(BCH_FS_HAVE_DELETED_SNAPSHOTS, &c->flags);
+err_create_lock:
+	up_write(&c->snapshot_create_lock);
 err:
 	darray_exit(&deleted_interior);
 	darray_exit(&deleted);

fs/bcachefs/super.c

@@ -720,6 +720,7 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 
 	mutex_init(&c->bio_bounce_pages_lock);
 	mutex_init(&c->snapshot_table_lock);
+	init_rwsem(&c->snapshot_create_lock);
 
 	spin_lock_init(&c->btree_write_error_lock);