btrfs: fix data race when accessing the last_trans field of a root

KCSAN complains about a data race when accessing the last_trans field of a
root:

  [  199.553628] BUG: KCSAN: data-race in btrfs_record_root_in_trans [btrfs] / record_root_in_trans [btrfs]

  [  199.555186] read to 0x000000008801e308 of 8 bytes by task 2812 on cpu 1:
  [  199.555210]  btrfs_record_root_in_trans+0x9a/0x128 [btrfs]
  [  199.555999]  start_transaction+0x154/0xcd8 [btrfs]
  [  199.556780]  btrfs_join_transaction+0x44/0x60 [btrfs]
  [  199.557559]  btrfs_dirty_inode+0x9c/0x140 [btrfs]
  [  199.558339]  btrfs_update_time+0x8c/0xb0 [btrfs]
  [  199.559123]  touch_atime+0x16c/0x1e0
  [  199.559151]  pipe_read+0x6a8/0x7d0
  [  199.559179]  vfs_read+0x466/0x498
  [  199.559204]  ksys_read+0x108/0x150
  [  199.559230]  __s390x_sys_read+0x68/0x88
  [  199.559257]  do_syscall+0x1c6/0x210
  [  199.559286]  __do_syscall+0xc8/0xf0
  [  199.559318]  system_call+0x70/0x98

  [  199.559431] write to 0x000000008801e308 of 8 bytes by task 2808 on cpu 0:
  [  199.559464]  record_root_in_trans+0x196/0x228 [btrfs]
  [  199.560236]  btrfs_record_root_in_trans+0xfe/0x128 [btrfs]
  [  199.561097]  start_transaction+0x154/0xcd8 [btrfs]
  [  199.561927]  btrfs_join_transaction+0x44/0x60 [btrfs]
  [  199.562700]  btrfs_dirty_inode+0x9c/0x140 [btrfs]
  [  199.563493]  btrfs_update_time+0x8c/0xb0 [btrfs]
  [  199.564277]  file_update_time+0xb8/0xf0
  [  199.564301]  pipe_write+0x8ac/0xab8
  [  199.564326]  vfs_write+0x33c/0x588
  [  199.564349]  ksys_write+0x108/0x150
  [  199.564372]  __s390x_sys_write+0x68/0x88
  [  199.564397]  do_syscall+0x1c6/0x210
  [  199.564424]  __do_syscall+0xc8/0xf0
  [  199.564452]  system_call+0x70/0x98

This is because we update and read last_trans concurrently without any
type of synchronization. This should be generally harmless and in the
worst case it can make us do extra locking (btrfs_record_root_in_trans())
trigger some warnings at ctree.c or do extra work during relocation - this
would probably only happen in case of load or store tearing.

So fix this by always reading and updating the field using READ_ONCE()
and WRITE_ONCE(), this silences KCSAN and prevents load and store tearing.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -321,7 +321,7 @@ int btrfs_copy_root(struct btrfs_trans_handle *trans,
 	WARN_ON(test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
 		trans->transid != fs_info->running_transaction->transid);
 	WARN_ON(test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
-		trans->transid != root->last_trans);
+		trans->transid != btrfs_get_root_last_trans(root));
 
 	level = btrfs_header_level(buf);
 	if (level == 0)
@@ -556,7 +556,7 @@ int btrfs_force_cow_block(struct btrfs_trans_handle *trans,
 	WARN_ON(test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
 		trans->transid != fs_info->running_transaction->transid);
 	WARN_ON(test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
-		trans->transid != root->last_trans);
+		trans->transid != btrfs_get_root_last_trans(root));
 
 	level = btrfs_header_level(buf);
 

fs/btrfs/ctree.h

@@ -356,6 +356,16 @@ static inline void btrfs_set_root_last_log_commit(struct btrfs_root *root, int c
 	WRITE_ONCE(root->last_log_commit, commit_id);
 }
 
+static inline u64 btrfs_get_root_last_trans(const struct btrfs_root *root)
+{
+	return READ_ONCE(root->last_trans);
+}
+
+static inline void btrfs_set_root_last_trans(struct btrfs_root *root, u64 transid)
+{
+	WRITE_ONCE(root->last_trans, transid);
+}
+
 /*
  * Structure that conveys information about an extent that is going to replace
  * all the extents in a file range.

fs/btrfs/defrag.c

@@ -139,7 +139,7 @@ int btrfs_add_inode_defrag(struct btrfs_trans_handle *trans,
 	if (trans)
 		transid = trans->transid;
 	else
-		transid = inode->root->last_trans;
+		transid = btrfs_get_root_last_trans(root);
 
 	defrag = kmem_cache_zalloc(btrfs_inode_defrag_cachep, GFP_NOFS);
 	if (!defrag)

fs/btrfs/disk-io.c

@@ -658,7 +658,7 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 	root->state = 0;
 	RB_CLEAR_NODE(&root->rb_node);
 
-	root->last_trans = 0;
+	btrfs_set_root_last_trans(root, 0);
 	root->free_objectid = 0;
 	root->nr_delalloc_inodes = 0;
 	root->nr_ordered_extents = 0;
@@ -1002,7 +1002,7 @@ int btrfs_add_log_tree(struct btrfs_trans_handle *trans,
 		return ret;
 	}
 
-	log_root->last_trans = trans->transid;
+	btrfs_set_root_last_trans(log_root, trans->transid);
 	log_root->root_key.offset = btrfs_root_id(root);
 
 	inode_item = &log_root->root_item.inode;

fs/btrfs/relocation.c

@@ -817,7 +817,7 @@ static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
 		goto abort;
 	}
 	set_bit(BTRFS_ROOT_SHAREABLE, &reloc_root->state);
-	reloc_root->last_trans = trans->transid;
+	btrfs_set_root_last_trans(reloc_root, trans->transid);
 	return reloc_root;
 fail:
 	kfree(root_item);
@@ -864,7 +864,7 @@ int btrfs_init_reloc_root(struct btrfs_trans_handle *trans,
 	 */
 	if (root->reloc_root) {
 		reloc_root = root->reloc_root;
-		reloc_root->last_trans = trans->transid;
+		btrfs_set_root_last_trans(reloc_root, trans->transid);
 		return 0;
 	}
 
@@ -1739,7 +1739,7 @@ static noinline_for_stack int merge_reloc_root(struct reloc_control *rc,
 		 * btrfs_update_reloc_root() and update our root item
 		 * appropriately.
 		 */
-		reloc_root->last_trans = trans->transid;
+		btrfs_set_root_last_trans(reloc_root, trans->transid);
 		trans->block_rsv = rc->block_rsv;
 
 		replaced = 0;
@@ -2082,7 +2082,7 @@ static int record_reloc_root_in_trans(struct btrfs_trans_handle *trans,
 	struct btrfs_root *root;
 	int ret;
 
-	if (reloc_root->last_trans == trans->transid)
+	if (btrfs_get_root_last_trans(reloc_root) == trans->transid)
 		return 0;
 
 	root = btrfs_get_fs_root(fs_info, reloc_root->root_key.offset, false);

fs/btrfs/transaction.c

@@ -405,7 +405,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
 	int ret = 0;
 
 	if ((test_bit(BTRFS_ROOT_SHAREABLE, &root->state) &&
-	    root->last_trans < trans->transid) || force) {
+	    btrfs_get_root_last_trans(root) < trans->transid) || force) {
 		WARN_ON(!force && root->commit_root != root->node);
 
 		/*
@@ -421,7 +421,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
 		smp_wmb();
 
 		spin_lock(&fs_info->fs_roots_radix_lock);
-		if (root->last_trans == trans->transid && !force) {
+		if (btrfs_get_root_last_trans(root) == trans->transid && !force) {
 			spin_unlock(&fs_info->fs_roots_radix_lock);
 			return 0;
 		}
@@ -429,7 +429,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
 				   (unsigned long)btrfs_root_id(root),
 				   BTRFS_ROOT_TRANS_TAG);
 		spin_unlock(&fs_info->fs_roots_radix_lock);
-		root->last_trans = trans->transid;
+		btrfs_set_root_last_trans(root, trans->transid);
 
 		/* this is pretty tricky.  We don't want to
 		 * take the relocation lock in btrfs_record_root_in_trans
@@ -491,7 +491,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 	 * and barriers
 	 */
 	smp_rmb();
-	if (root->last_trans == trans->transid &&
+	if (btrfs_get_root_last_trans(root) == trans->transid &&
 	    !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
 		return 0;