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BUG: KCSAN: data-race in btrfs_update_block_group / caching_thread

write to 0xffff88802133e4a4 of 4 bytes by task 58 on cpu 0:
 caching_thread+0x799/0x8d0 fs/btrfs/block-group.c:691
 btrfs_work_helper+0x1e2/0x380 fs/btrfs/async-thread.c:318
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff88802133e4a4 of 4 bytes by task 15722 on cpu 1:
 btrfs_block_group_done fs/btrfs/block-group.h:292 [inline]
 btrfs_update_block_group+0x142/0x960 fs/btrfs/block-group.c:2864
 __btrfs_free_extent+0xcf0/0x16d0 fs/btrfs/extent-tree.c:3233
 run_delayed_tree_ref+0x1ae/0x2e0 fs/btrfs/extent-tree.c:1689
 run_one_delayed_ref fs/btrfs/extent-tree.c:1713 [inline]
 btrfs_run_delayed_refs_for_head fs/btrfs/extent-tree.c:1973 [inline]
 __btrfs_run_delayed_refs+0x5e1/0xab0 fs/btrfs/extent-tree.c:2038
 btrfs_run_delayed_refs+0xcf/0x320 fs/btrfs/extent-tree.c:2169
 commit_cowonly_roots+0xec/0x5e0 fs/btrfs/transaction.c:1233
 btrfs_commit_transaction+0x95e/0x1870 fs/btrfs/transaction.c:2275
 btrfs_commit_super+0x8f/0xa0 fs/btrfs/disk-io.c:4076
 close_ctree+0x1af/0x4ad fs/btrfs/disk-io.c:4140
 btrfs_put_super+0x21/0x30 fs/btrfs/super.c:326
 generic_shutdown_super+0xde/0x210 fs/super.c:464
 kill_anon_super+0x23/0x40 fs/super.c:1055
 btrfs_kill_super+0x26/0x40 fs/btrfs/super.c:2347
 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:140
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:174 [inline]
 exit_to_user_mode_prepare+0x17c/0x1b0 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:283 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:294
 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: 15722 Comm: syz-executor.5 Not tainted 5.11.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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