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BUG: KCSAN: data-race in caching_thread / find_free_extent

write to 0xffff88802d33fca4 of 4 bytes by task 8 on cpu 1:
 caching_thread+0x73a/0x870 fs/btrfs/block-group.c:683
 btrfs_work_helper+0x1e2/0x380 fs/btrfs/async-thread.c:318
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff88802d33fca4 of 4 bytes by task 8456 on cpu 0:
 find_free_extent+0xf51/0x1a30 fs/btrfs/extent-tree.c:4183
 btrfs_reserve_extent+0x118/0x2c0 fs/btrfs/extent-tree.c:4317
 btrfs_alloc_tree_block+0xfe/0x870 fs/btrfs/extent-tree.c:4718
 alloc_tree_block_no_bg_flush fs/btrfs/ctree.c:989 [inline]
 __btrfs_cow_block+0x2f2/0xac0 fs/btrfs/ctree.c:1045
 btrfs_cow_block+0x252/0x3a0 fs/btrfs/ctree.c:1490
 commit_cowonly_roots+0x91/0x5d0 fs/btrfs/transaction.c:1225
 btrfs_commit_transaction+0x95f/0x1810 fs/btrfs/transaction.c:2283
 btrfs_sync_fs+0x1ff/0x340 fs/btrfs/super.c:1424
 __sync_filesystem fs/sync.c:39 [inline]
 sync_filesystem+0x112/0x160 fs/sync.c:67
 generic_shutdown_super+0x3c/0x1f0 fs/super.c:448
 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+0x16b/0x1a0 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:302
 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: 8456 Comm: syz-executor.4 Not tainted 5.11.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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