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BUG: KCSAN: data-race in __writeback_single_inode / __xa_set_mark

write to 0xffff8881205fd76c of 4 bytes by task 8282 on cpu 0:
 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:610
 mark_buffer_dirty+0x12e/0x260 fs/buffer.c:1138
 __block_commit_write fs/buffer.c:2085 [inline]
 block_write_end+0x13d/0x220 fs/buffer.c:2163
 generic_write_end+0x5c/0x220 fs/buffer.c:2177
 ext4_da_write_end+0x57e/0x760 fs/ext4/inode.c:3092
 generic_perform_write+0x23b/0x390 mm/filemap.c:3310
 ext4_buffered_write_iter+0x2cc/0x3b0 fs/ext4/file.c:270
 ext4_file_write_iter+0x76f/0x1010 include/linux/fs.h:804
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write fs/read_write.c:484 [inline]
 __vfs_write+0x2d8/0x340 fs/read_write.c:497
 vfs_write+0x19b/0x350 fs/read_write.c:559
 ksys_write+0xce/0x180 fs/read_write.c:612
 __do_sys_write fs/read_write.c:624 [inline]
 __se_sys_write fs/read_write.c:621 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:621
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881205fd76c of 4 bytes by task 7 on cpu 1:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:523 [inline]
 __writeback_single_inode+0x273/0x610 fs/fs-writeback.c:1502
 writeback_sb_inodes+0x5ac/0xe40 fs/fs-writeback.c:1716
 __writeback_inodes_wb+0xb2/0x240 fs/fs-writeback.c:1785
 wb_writeback+0x2e0/0x730 fs/fs-writeback.c:1894
 wb_check_background_flush fs/fs-writeback.c:1962 [inline]
 wb_do_writeback+0x5cc/0x610 fs/fs-writeback.c:2050
 wb_workfn+0xc8/0x460 fs/fs-writeback.c:2078
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:291
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:293

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7 Comm: kworker/u4:0 Not tainted 5.8.0-rc4-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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