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BUG: KCSAN: data-race in btrfs_update_block_group / caching_thread

write to 0xffff8880af1018a4 of 4 bytes by task 20292 on cpu 1:
 caching_thread+0x691/0x7c0 fs/btrfs/block-group.c:649
 btrfs_work_helper+0x1e2/0x380 fs/btrfs/async-thread.c:318
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff8880af1018a4 of 4 bytes by task 8449 on cpu 0:
 btrfs_block_group_done fs/btrfs/block-group.h:290 [inline]
 btrfs_update_block_group+0x142/0x900 fs/btrfs/block-group.c:2872
 __btrfs_free_extent+0xd11/0x16f0 fs/btrfs/extent-tree.c:3299
 run_delayed_tree_ref+0x1a8/0x2e0 fs/btrfs/extent-tree.c:1690
 run_one_delayed_ref fs/btrfs/extent-tree.c:1714 [inline]
 btrfs_run_delayed_refs_for_head fs/btrfs/extent-tree.c:1974 [inline]
 __btrfs_run_delayed_refs+0x5b1/0xa40 fs/btrfs/extent-tree.c:2039
 btrfs_run_delayed_refs+0x206/0x320 fs/btrfs/extent-tree.c:2189
 commit_cowonly_roots+0xe9/0x5d0 fs/btrfs/transaction.c:1194
 btrfs_commit_transaction+0xaee/0x1910 fs/btrfs/transaction.c:2283
 btrfs_commit_super+0x8f/0xa0 fs/btrfs/disk-io.c:3964
 close_ctree+0x1af/0x49b fs/btrfs/disk-io.c:4028
 btrfs_put_super+0x21/0x30 fs/btrfs/super.c:321
 generic_shutdown_super+0xcd/0x1f0 fs/super.c:464
 kill_anon_super+0x23/0x40 fs/super.c:1108
 btrfs_kill_super+0x26/0x40 fs/btrfs/super.c:2263
 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:151
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:164 [inline]
 exit_to_user_mode_prepare+0x13c/0x170 kernel/entry/common.c:191
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:266
 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: 0 PID: 8449 Comm: syz-executor.0 Not tainted 5.10.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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