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BUG: KCSAN: data-race in __xa_set_mark / writeback_sb_inodes

write to 0xffff88811edbe3ac of 4 bytes by task 24264 on cpu 1:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:886 [inline]
 __xa_set_mark+0x317/0x330 lib/xarray.c:1708
 __set_page_dirty+0x17e/0x1d0 fs/buffer.c:609
 mark_buffer_dirty+0x12e/0x260 fs/buffer.c:1137
 __block_commit_write fs/buffer.c:2084 [inline]
 block_commit_write fs/buffer.c:2476 [inline]
 block_page_mkwrite+0x25d/0x2e0 fs/buffer.c:2525
 ext4_page_mkwrite+0x572/0xae0 fs/ext4/inode.c:5985
 do_page_mkwrite mm/memory.c:2730 [inline]
 wp_page_shared+0x2d8/0x820 mm/memory.c:3059
 do_wp_page+0x6eb/0x740 mm/memory.c:3152
 handle_pte_fault mm/memory.c:4418 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x1899/0x1af0 mm/memory.c:4633
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff88811edbe3ac of 4 bytes by task 21 on cpu 0:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:493 [inline]
 requeue_inode fs/fs-writeback.c:1409 [inline]
 writeback_sb_inodes+0x958/0xff0 fs/fs-writeback.c:1748
 wb_writeback+0x28d/0x6f0 fs/fs-writeback.c:1894
 wb_do_writeback+0x101/0x5f0 fs/fs-writeback.c:2039
 wb_workfn+0xc8/0x470 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 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: 21 Comm: kworker/u4:1 Not tainted 5.9.0-rc7-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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