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BUG: KCSAN: data-race in __writeback_single_inode / xas_set_mark

write to 0xffff88811cc2c36c of 4 bytes by task 14480 on cpu 1:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:886 [inline]
 xas_set_mark+0x140/0x150 lib/xarray.c:870
 __xa_set_mark+0xdd/0x110 lib/xarray.c:1708
 __set_page_dirty+0x151/0x270 fs/buffer.c:618
 __set_page_dirty_buffers+0x1ca/0x280 fs/buffer.c:677
 ext4_set_page_dirty+0xba/0x180 fs/ext4/inode.c:3603
 set_page_dirty+0x11b/0x2a0 mm/page-writeback.c:2572
 fault_dirty_shared_page+0x74/0x1c0 mm/memory.c:2559
 wp_page_shared mm/memory.c:2872 [inline]
 do_wp_page+0xa71/0xca0 mm/memory.c:2973
 handle_pte_fault mm/memory.c:4231 [inline]
 __handle_mm_fault+0x2d96/0x2da0 mm/memory.c:4345
 handle_mm_fault+0x21c/0x540 mm/memory.c:4382
 do_user_addr_fault arch/x86/mm/fault.c:1464 [inline]
 do_page_fault+0x48a/0xa96 arch/x86/mm/fault.c:1535
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203

read to 0xffff88811cc2c36c of 4 bytes by task 7 on cpu 0:
 mapping_tagged include/linux/fs.h:521 [inline]
 __writeback_single_inode+0x376/0x910 fs/fs-writeback.c:1502
 writeback_sb_inodes+0x4c8/0xa10 fs/fs-writeback.c:1716
 wb_writeback+0x1f9/0x6a0 fs/fs-writeback.c:1892
 wb_do_writeback fs/fs-writeback.c:2037 [inline]
 wb_workfn+0x215/0x970 fs/fs-writeback.c:2078
 process_one_work+0x424/0x930 kernel/workqueue.c:2268
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2414
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.7.0-rc1-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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