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BUG: KCSAN: data-race in __xa_set_mark / writeback_sb_inodes

write to 0xffff888102a18434 of 4 bytes by task 23537 on cpu 1:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:889 [inline]
 __xa_set_mark+0x279/0x290 lib/xarray.c:1899
 __set_page_dirty+0x155/0x1a0 fs/buffer.c:609
 mark_buffer_dirty+0x126/0x200 fs/buffer.c:1137
 __block_commit_write fs/buffer.c:2084 [inline]
 block_commit_write fs/buffer.c:2477 [inline]
 block_page_mkwrite+0x26d/0x320 fs/buffer.c:2526
 ext4_page_mkwrite+0x726/0xf20 fs/ext4/inode.c:6087
 do_page_mkwrite mm/memory.c:2720 [inline]
 wp_page_shared mm/memory.c:3050 [inline]
 do_wp_page+0x86c/0xba0 mm/memory.c:3151
 handle_pte_fault mm/memory.c:4336 [inline]
 __handle_mm_fault mm/memory.c:4453 [inline]
 handle_mm_fault+0xb31/0x1a70 mm/memory.c:4551
 do_user_addr_fault+0x60c/0xc00 arch/x86/mm/fault.c:1390
 handle_page_fault arch/x86/mm/fault.c:1475 [inline]
 exc_page_fault+0x94/0x230 arch/x86/mm/fault.c:1531
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:577

read to 0xffff888102a18434 of 4 bytes by task 1786 on cpu 0:
 xa_marked include/linux/xarray.h:418 [inline]
 mapping_tagged include/linux/fs.h:490 [inline]
 requeue_inode fs/fs-writeback.c:1409 [inline]
 writeback_sb_inodes+0x822/0xf10 fs/fs-writeback.c:1759
 wb_writeback+0x28e/0x700 fs/fs-writeback.c:1905
 wb_do_writeback+0x101/0x510 fs/fs-writeback.c:2050
 wb_workfn+0xc8/0x520 fs/fs-writeback.c:2091
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2275
 worker_thread+0x636/0xae0 kernel/workqueue.c:2421
 kthread+0x1d0/0x1f0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 1786 Comm: kworker/u4:5 Tainted: G        W         5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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