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BUG: KCSAN: data-race in __xa_set_mark / writeback_sb_inodes

write to 0xffff88810bdb9d84 of 4 bytes by task 8069 on cpu 0:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:889 [inline]
 __xa_set_mark+0x166/0x190 lib/xarray.c:1898
 __set_page_dirty+0x17e/0x1d0 fs/buffer.c:609
 mark_buffer_dirty+0x126/0x230 fs/buffer.c:1137
 __block_commit_write fs/buffer.c:2084 [inline]
 block_write_end+0x13d/0x230 fs/buffer.c:2162
 generic_write_end+0x5b/0x250 fs/buffer.c:2176
 ext4_da_write_end+0x59b/0x760 fs/ext4/inode.c:3100
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3322
 ext4_buffered_write_iter+0x2e5/0x3d0 fs/ext4/file.c:270
 ext4_file_write_iter+0x45e/0x1090 fs/ext4/file.c:503
 call_write_iter include/linux/fs.h:1887 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x617/0x690 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88810bdb9d84 of 4 bytes by task 2969 on cpu 1:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:488 [inline]
 requeue_inode fs/fs-writeback.c:1409 [inline]
 writeback_sb_inodes+0x91d/0x1020 fs/fs-writeback.c:1748
 __writeback_inodes_wb+0xb0/0x2a0 fs/fs-writeback.c:1790
 wb_writeback+0x290/0x660 fs/fs-writeback.c:1896
 wb_check_background_flush fs/fs-writeback.c:1964 [inline]
 wb_do_writeback+0x583/0x5d0 fs/fs-writeback.c:2052
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/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: 2969 Comm: kworker/u4:3 Not tainted 5.10.0-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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