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BUG: KCSAN: data-race in caching_thread / find_free_extent

write to 0xffff8880bbb408a4 of 4 bytes by task 16537 on cpu 0:
 caching_thread+0x6ac/0x7e0 fs/btrfs/block-group.c:649
 btrfs_work_helper+0x1e2/0x3c0 fs/btrfs/async-thread.c:318
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff8880bbb408a4 of 4 bytes by task 10409 on cpu 1:
 btrfs_block_group_done fs/btrfs/block-group.h:290 [inline]
 find_free_extent+0xc9e/0x1a70 fs/btrfs/extent-tree.c:4051
 btrfs_reserve_extent+0x118/0x2c0 fs/btrfs/extent-tree.c:4206
 btrfs_alloc_tree_block+0x101/0x800 fs/btrfs/extent-tree.c:4603
 alloc_tree_block_no_bg_flush fs/btrfs/ctree.c:987 [inline]
 __btrfs_cow_block+0x2d3/0x9e0 fs/btrfs/ctree.c:1042
 btrfs_cow_block+0x261/0x3d0 fs/btrfs/ctree.c:1487
 btrfs_search_slot+0x722/0x12e0 fs/btrfs/ctree.c:2745
 btrfs_lookup_inode+0x5b/0x1d0 fs/btrfs/inode-item.c:414
 __btrfs_update_delayed_inode+0xee/0x6b0 fs/btrfs/delayed-inode.c:1034
 btrfs_update_delayed_inode fs/btrfs/delayed-inode.c:1113 [inline]
 __btrfs_commit_inode_delayed_items+0x4a0/0x4c0 fs/btrfs/delayed-inode.c:1133
 __btrfs_run_delayed_items+0x199/0x480 fs/btrfs/delayed-inode.c:1168
 btrfs_run_delayed_items+0x16/0x20 fs/btrfs/delayed-inode.c:1192
 btrfs_commit_transaction+0x575/0x18e0 fs/btrfs/transaction.c:2154
 btrfs_sync_fs+0x14b/0x240 fs/btrfs/super.c:1383
 __sync_filesystem fs/sync.c:39 [inline]
 sync_filesystem+0x112/0x160 fs/sync.c:67
 generic_shutdown_super+0x42/0x210 fs/super.c:448
 kill_anon_super+0x23/0x40 fs/super.c:1108
 btrfs_kill_super+0x26/0x40 fs/btrfs/super.c:2265
 deactivate_locked_super+0x6a/0xb0 fs/super.c:335
 deactivate_super+0x85/0x90 fs/super.c:366
 cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
 task_work_run+0x8e/0x110 kernel/task_work.c:141
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:165 [inline]
 exit_to_user_mode_prepare+0x1b2/0x210 kernel/entry/common.c:192
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:267
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10409 Comm: syz-executor.2 Not tainted 5.9.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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