Btrfs: fix race between transaction commit and empty block group removal

fdmanana · masoncl · commit d4b450cd4b33 · 2015-02-02T19:24:48.000-08:00
Committing a transaction can race with automatic removal of empty block groups (cleaner kthread), leading to a BUG_ON() in the transaction commit code while running btrfs_finish_extent_commit(). The following sequence diagram shows how it can happen: CPU 1 CPU 2 btrfs_commit_transaction() fs_info->running_transaction = NULL btrfs_finish_extent_commit() find_first_extent_bit() -> found range for block group X in fs_info->freed_extents[] btrfs_delete_unused_bgs() -> found block group X Removed block group X's range from fs_info->freed_extents[] btrfs_remove_chunk() btrfs_remove_block_group(bg X) unpin_extent_range(bg X range) btrfs_lookup_block_group(bg X) -> returns NULL -> BUG_ON() The trace that results from the BUG_ON() is: [48665.187808] ------------[ cut here ]------------ [48665.188032] kernel BUG at fs/btrfs/extent-tree.c:5675! [48665.188032] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC [48665.188032] Modules linked in: dm_flakey dm_mod crc32c_generic btrfs xor raid6_pq nfsd auth_rpcgss oid_registry nfs_acl nfs lockd grace fscache sunrpc loop parport_pc evdev microcode [48665.197388] CPU: 2 PID: 31211 Comm: kworker/u32:16 Tainted: G W 3.19.0-rc5-btrfs-next-4+ #1 [48665.197388] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014 [48665.197388] Workqueue: events_unbound btrfs_async_reclaim_metadata_space [btrfs] [48665.197388] task: ffff880222011810 ti: ffff8801b56a4000 task.ti: ffff8801b56a4000 [48665.197388] RIP: 0010:[<ffffffffa0350d05>] [<ffffffffa0350d05>] unpin_extent_range+0x6a/0x1ba [btrfs] [48665.197388] RSP: 0018:ffff8801b56a7b88 EFLAGS: 00010246 [48665.197388] RAX: 0000000000000000 RBX: ffff8802143a6000 RCX: ffff8802220120c8 [48665.197388] RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff8800a3c140b0 [48665.197388] RBP: ffff8801b56a7bd8 R08: 0000000000000003 R09: 0000000000000000 [48665.197388] R10: 0000000000000000 R11: 000000000000bbac R12: 0000000012e8e000 [48665.197388] R13: ffff8800a3c14000 R14: 0000000000000000 R15: 0000000000000000 [48665.197388] FS: 0000000000000000(0000) GS:ffff88023ec40000(0000) knlGS:0000000000000000 [48665.197388] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b [48665.197388] CR2: 00007f065e42f270 CR3: 0000000206f70000 CR4: 00000000000006e0 [48665.197388] Stack: [48665.197388] ffff8801b56a7bd8 0000000012ea0000 01ff8800a3c14138 0000000012e9ffff [48665.197388] ffff880141df3dd8 ffff8802143a6000 ffff8800a3c14138 ffff880141df3df0 [48665.197388] ffff880141df3dd8 0000000000000000 ffff8801b56a7c08 ffffffffa0354227 [48665.197388] Call Trace: [48665.197388] [<ffffffffa0354227>] btrfs_finish_extent_commit+0xb0/0xd9 [btrfs] [48665.197388] [<ffffffffa0366b4b>] btrfs_commit_transaction+0x791/0x92c [btrfs] [48665.197388] [<ffffffffa0352432>] flush_space+0x43d/0x452 [btrfs] [48665.197388] [<ffffffff814295c3>] ? _raw_spin_unlock+0x28/0x33 [48665.197388] [<ffffffffa035255f>] btrfs_async_reclaim_metadata_space+0x118/0x164 [btrfs] [48665.197388] [<ffffffff81059917>] ? process_one_work+0x14b/0x3ab [48665.197388] [<ffffffff810599ac>] process_one_work+0x1e0/0x3ab [48665.197388] [<ffffffff81079fa9>] ? trace_hardirqs_off+0xd/0xf [48665.197388] [<ffffffff8105a55b>] worker_thread+0x210/0x2d0 [48665.197388] [<ffffffff8105a34b>] ? rescuer_thread+0x2c3/0x2c3 [48665.197388] [<ffffffff8105e5c0>] kthread+0xef/0xf7 [48665.197388] [<ffffffff81429682>] ? _raw_spin_unlock_irq+0x2d/0x39 [48665.197388] [<ffffffff8105e4d1>] ? __kthread_parkme+0xad/0xad [48665.197388] [<ffffffff81429dec>] ret_from_fork+0x7c/0xb0 [48665.197388] [<ffffffff8105e4d1>] ? __kthread_parkme+0xad/0xad [48665.197388] Code: 85 f6 74 14 49 8b 06 49 03 46 09 49 39 c4 72 1d 4c 89 f7 e8 83 ec ff ff 4c 89 e6 4c 89 ef e8 1e f1 ff ff 48 85 c0 49 89 c6 75 02 <0f> 0b 49 8b 1e 49 03 5e 09 48 8b [48665.197388] RIP [<ffffffffa0350d05>] unpin_extent_range+0x6a/0x1ba [btrfs] [48665.197388] RSP <ffff8801b56a7b88> [48665.272246] ---[ end trace b9c6ab9957521376 ]--- Fix this by ensuring that unpining the block group's range in btrfs_finish_extent_commit() is done in a synchronized fashion with removing the block group's range from freed_extents[] in btrfs_delete_unused_bgs() This race got introduced with the change: Btrfs: remove empty block groups automatically commit 47ab2a6 Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
@@ -1744,6 +1744,7 @@ struct btrfs_fs_info {
 
 	spinlock_t unused_bgs_lock;
 	struct list_head unused_bgs;
+	struct mutex unused_bg_unpin_mutex;
 
 	/* For btrfs to record security options */
 	struct security_mnt_opts security_opts;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
@@ -2242,6 +2242,7 @@ int open_ctree(struct super_block *sb,
 	spin_lock_init(&fs_info->qgroup_op_lock);
 	spin_lock_init(&fs_info->buffer_lock);
 	spin_lock_init(&fs_info->unused_bgs_lock);
+	mutex_init(&fs_info->unused_bg_unpin_mutex);
 	rwlock_init(&fs_info->tree_mod_log_lock);
 	mutex_init(&fs_info->reloc_mutex);
 	mutex_init(&fs_info->delalloc_root_mutex);
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
@@ -5735,17 +5735,21 @@ int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans,
 		unpin = &fs_info->freed_extents[0];
 
 	while (1) {
+		mutex_lock(&fs_info->unused_bg_unpin_mutex);
 		ret = find_first_extent_bit(unpin, 0, &start, &end,
 					    EXTENT_DIRTY, NULL);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&fs_info->unused_bg_unpin_mutex);
 			break;
+		}
 
 		if (btrfs_test_opt(root, DISCARD))
 			ret = btrfs_discard_extent(root, start,
 						   end + 1 - start, NULL);
 
 		clear_extent_dirty(unpin, start, end, GFP_NOFS);
 		unpin_extent_range(root, start, end, true);
+		mutex_unlock(&fs_info->unused_bg_unpin_mutex);
 		cond_resched();
 	}
 
@@ -9561,18 +9565,33 @@ void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info)
 		 */
 		start = block_group->key.objectid;
 		end = start + block_group->key.offset - 1;
+		/*
+		 * Hold the unused_bg_unpin_mutex lock to avoid racing with
+		 * btrfs_finish_extent_commit(). If we are at transaction N,
+		 * another task might be running finish_extent_commit() for the
+		 * previous transaction N - 1, and have seen a range belonging
+		 * to the block group in freed_extents[] before we were able to
+		 * clear the whole block group range from freed_extents[]. This
+		 * means that task can lookup for the block group after we
+		 * unpinned it from freed_extents[] and removed it, leading to
+		 * a BUG_ON() at btrfs_unpin_extent_range().
+		 */
+		mutex_lock(&fs_info->unused_bg_unpin_mutex);
 		ret = clear_extent_bits(&fs_info->freed_extents[0], start, end,
 				  EXTENT_DIRTY, GFP_NOFS);
 		if (ret) {
+			mutex_unlock(&fs_info->unused_bg_unpin_mutex);
 			btrfs_set_block_group_rw(root, block_group);
 			goto end_trans;
 		}
 		ret = clear_extent_bits(&fs_info->freed_extents[1], start, end,
 				  EXTENT_DIRTY, GFP_NOFS);
 		if (ret) {
+			mutex_unlock(&fs_info->unused_bg_unpin_mutex);
 			btrfs_set_block_group_rw(root, block_group);
 			goto end_trans;
 		}
+		mutex_unlock(&fs_info->unused_bg_unpin_mutex);
 
 		/* Reset pinned so btrfs_put_block_group doesn't complain */
 		block_group->pinned = 0;
