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BUG: KCSAN: data-race in __writeback_single_inode / __xa_set_mark

write to 0xffff88814046377c of 4 bytes by task 30572 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:889 [inline]
 __xa_set_mark+0x187/0x1b0 lib/xarray.c:1898
 __set_page_dirty+0x17e/0x1d0 fs/buffer.c:609
 mark_buffer_dirty+0x126/0x230 fs/buffer.c:1137
 mark_buffer_dirty_inode+0x48/0x1b0 fs/buffer.c:575
 fat_zeroed_cluster+0x215/0x5e0 fs/fat/dir.c:1102
 fat_alloc_new_dir+0x43f/0x4b0 fs/fat/dir.c:1185
 vfat_mkdir+0x70/0x210 fs/fat/namei_vfat.c:860
 vfs_mkdir+0x288/0x350 fs/namei.c:3652
 do_mkdirat+0x132/0x220 fs/namei.c:3675
 __do_sys_mkdirat fs/namei.c:3686 [inline]
 __se_sys_mkdirat fs/namei.c:3684 [inline]
 __x64_sys_mkdirat+0x3e/0x50 fs/namei.c:3684
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88814046377c of 4 bytes by task 3055 on cpu 1:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:488 [inline]
 __writeback_single_inode+0x23a/0x500 fs/fs-writeback.c:1511
 writeback_sb_inodes+0x6a0/0x1020 fs/fs-writeback.c:1723
 __writeback_inodes_wb+0xb0/0x2a0 fs/fs-writeback.c:1792
 wb_writeback+0x290/0x660 fs/fs-writeback.c:1898
 wb_check_start_all fs/fs-writeback.c:2022 [inline]
 wb_do_writeback+0x3f9/0x5d0 fs/fs-writeback.c:2048
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2082
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x64a/0xaa0 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 3055 Comm: kworker/u4:4 Not tainted 5.11.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-7:3)
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