btrfs: Fix race condition between delayed refs and blockgroup removal

When the delayed refs for a head are all run, eventually
cleanup_ref_head is called which (in case of deletion) obtains a
reference for the relevant btrfs_space_info struct by querying the bg
for the range. This is problematic because when the last extent of a
bg is deleted a race window emerges between removal of that bg and the
subsequent invocation of cleanup_ref_head. This can result in cache being null
and either a null pointer dereference or assertion failure.

	task: ffff8d04d31ed080 task.stack: ffff9e5dc10cc000
	RIP: 0010:assfail.constprop.78+0x18/0x1a [btrfs]
	RSP: 0018:ffff9e5dc10cfbe8 EFLAGS: 00010292
	RAX: 0000000000000044 RBX: 0000000000000000 RCX: 0000000000000000
	RDX: ffff8d04ffc1f868 RSI: ffff8d04ffc178c8 RDI: ffff8d04ffc178c8
	RBP: ffff8d04d29e5ea0 R08: 00000000000001f0 R09: 0000000000000001
	R10: ffff9e5dc0507d58 R11: 0000000000000001 R12: ffff8d04d29e5ea0
	R13: ffff8d04d29e5f08 R14: ffff8d04efe29b40 R15: ffff8d04efe203e0
	FS:  00007fbf58ead500(0000) GS:ffff8d04ffc00000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 00007fe6c6975648 CR3: 0000000013b2a000 CR4: 00000000000006f0
	DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
	DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
	Call Trace:
	 __btrfs_run_delayed_refs+0x10e7/0x12c0 [btrfs]
	 btrfs_run_delayed_refs+0x68/0x250 [btrfs]
	 btrfs_should_end_transaction+0x42/0x60 [btrfs]
	 btrfs_truncate_inode_items+0xaac/0xfc0 [btrfs]
	 btrfs_evict_inode+0x4c6/0x5c0 [btrfs]
	 evict+0xc6/0x190
	 do_unlinkat+0x19c/0x300
	 do_syscall_64+0x74/0x140
	 entry_SYSCALL_64_after_hwframe+0x3d/0xa2
	RIP: 0033:0x7fbf589c57a7

To fix this, introduce a new flag "is_system" to head_ref structs,
which is populated at insertion time. This allows to decouple the
querying for the spaceinfo from querying the possibly deleted bg.

Fixes: d7eae3403f46 ("Btrfs: rework delayed ref total_bytes_pinned accounting")
CC: stable@vger.kernel.org # 4.14+
Suggested-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index 9e98295de7ce..e1b0651686f7 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -540,8 +540,10 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
 		     struct btrfs_delayed_ref_head *head_ref,
 		     struct btrfs_qgroup_extent_record *qrecord,
 		     u64 bytenr, u64 num_bytes, u64 ref_root, u64 reserved,
-		     int action, int is_data, int *qrecord_inserted_ret,
+		     int action, int is_data, int is_system,
+		     int *qrecord_inserted_ret,
 		     int *old_ref_mod, int *new_ref_mod)
+
 {
 	struct btrfs_delayed_ref_head *existing;
 	struct btrfs_delayed_ref_root *delayed_refs;
@@ -585,6 +587,7 @@ add_delayed_ref_head(struct btrfs_fs_info *fs_info,
 	head_ref->ref_mod = count_mod;
 	head_ref->must_insert_reserved = must_insert_reserved;
 	head_ref->is_data = is_data;
+	head_ref->is_system = is_system;
 	head_ref->ref_tree = RB_ROOT;
 	INIT_LIST_HEAD(&head_ref->ref_add_list);
 	RB_CLEAR_NODE(&head_ref->href_node);
@@ -772,6 +775,7 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
 	struct btrfs_delayed_ref_root *delayed_refs;
 	struct btrfs_qgroup_extent_record *record = NULL;
 	int qrecord_inserted;
+	int is_system = (ref_root == BTRFS_CHUNK_TREE_OBJECTID);
 
 	BUG_ON(extent_op && extent_op->is_data);
 	ref = kmem_cache_alloc(btrfs_delayed_tree_ref_cachep, GFP_NOFS);
@@ -800,8 +804,8 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
 	 */
 	head_ref = add_delayed_ref_head(fs_info, trans, head_ref, record,
 					bytenr, num_bytes, 0, 0, action, 0,
-					&qrecord_inserted, old_ref_mod,
-					new_ref_mod);
+					is_system, &qrecord_inserted,
+					old_ref_mod, new_ref_mod);
 
 	add_delayed_tree_ref(fs_info, trans, head_ref, &ref->node, bytenr,
 			     num_bytes, parent, ref_root, level, action);
@@ -868,7 +872,7 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
 	 */
 	head_ref = add_delayed_ref_head(fs_info, trans, head_ref, record,
 					bytenr, num_bytes, ref_root, reserved,
-					action, 1, &qrecord_inserted,
+					action, 1, 0, &qrecord_inserted,
 					old_ref_mod, new_ref_mod);
 
 	add_delayed_data_ref(fs_info, trans, head_ref, &ref->node, bytenr,
@@ -898,9 +902,14 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
 	delayed_refs = &trans->transaction->delayed_refs;
 	spin_lock(&delayed_refs->lock);
 
+	/*
+	 * extent_ops just modify the flags of an extent and they don't result
+	 * in ref count changes, hence it's safe to pass false/0 for is_system
+	 * argument
+	 */
 	add_delayed_ref_head(fs_info, trans, head_ref, NULL, bytenr,
 			     num_bytes, 0, 0, BTRFS_UPDATE_DELAYED_HEAD,
-			     extent_op->is_data, NULL, NULL, NULL);
+			     extent_op->is_data, 0, NULL, NULL, NULL);
 
 	spin_unlock(&delayed_refs->lock);
 	return 0;