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BUG: KCSAN: data-race in __xa_set_mark / ext4_writepages

write to 0xffff888113b716b4 of 4 bytes by task 18939 on cpu 1:
 xa_mark_set lib/xarray.c:69 [inline]
 xas_set_mark lib/xarray.c:889 [inline]
 __xa_set_mark+0x2d9/0x2f0 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
 ext4_write_end+0x185/0x9f0 fs/ext4/inode.c:1283
 ext4_da_write_end+0x91/0x750 fs/ext4/inode.c:3060
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3536
 ext4_buffered_write_iter+0x2ce/0x3b0 fs/ext4/file.c:269
 ext4_file_write_iter+0x480/0x10b0 fs/ext4/file.c:501
 call_write_iter include/linux/fs.h:1887 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x630/0x6b0 fs/read_write.c:586
 ksys_write+0xce/0x180 fs/read_write.c:639
 __do_sys_write fs/read_write.c:651 [inline]
 __se_sys_write fs/read_write.c:648 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:648
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888113b716b4 of 4 bytes by task 725 on cpu 0:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:488 [inline]
 ext4_writepages+0x158/0x1e20 fs/ext4/inode.c:2649
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x560 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6a0/0x1020 fs/fs-writeback.c:1721
 wb_writeback+0x27d/0x660 fs/fs-writeback.c:1894
 wb_do_writeback+0x101/0x5d0 fs/fs-writeback.c:2039
 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+0x1ff/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 725 Comm: kworker/u4:3 Not tainted 5.9.0-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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