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BUG: KCSAN: data-race in writeback_sb_inodes / xas_set_mark

write to 0xffff888128fa3ad4 of 4 bytes by task 25665 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:1706
 __set_page_dirty+0x151/0x270 fs/buffer.c:624
 mark_buffer_dirty+0x26e/0x2b0 fs/buffer.c:1152
 __block_commit_write.isra.0+0x11d/0x170 fs/buffer.c:2081
 block_commit_write fs/buffer.c:2473 [inline]
 block_page_mkwrite+0x21d/0x230 fs/buffer.c:2522
 ext4_page_mkwrite+0x4e8/0x800 fs/ext4/inode.c:5933
 do_page_mkwrite+0xfa/0x210 mm/memory.c:2364
 wp_page_shared mm/memory.c:2691 [inline]
 do_wp_page+0xaa3/0xcc0 mm/memory.c:2803
 handle_pte_fault mm/memory.c:4049 [inline]
 __handle_mm_fault+0x1c5e/0x2cf0 mm/memory.c:4163
 handle_mm_fault+0x21c/0x540 mm/memory.c:4200
 do_user_addr_fault arch/x86/mm/fault.c:1442 [inline]
 do_page_fault+0x4a4/0xa52 arch/x86/mm/fault.c:1517
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203

read to 0xffff888128fa3ad4 of 4 bytes by task 864 on cpu 0:
 mapping_tagged include/linux/fs.h:521 [inline]
 requeue_inode fs/fs-writeback.c:1401 [inline]
 writeback_sb_inodes+0x6c9/0xa10 fs/fs-writeback.c:1743
 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:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 864 Comm: kworker/u4:5 Not tainted 5.6.0-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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