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BUG: KCSAN: data-race in __xa_set_mark / writeback_sb_inodes

write to 0xffff88812a2b02bc of 4 bytes by task 8726 on cpu 1:
 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:609
 mark_buffer_dirty+0x12e/0x260 fs/buffer.c:1137
 __ext4_handle_dirty_metadata+0x20a/0x5a0 fs/ext4/ext4_jbd2.c:362
 ext4_do_update_inode fs/ext4/inode.c:5072 [inline]
 ext4_mark_iloc_dirty+0x1302/0x1660 fs/ext4/inode.c:5624
 __ext4_mark_inode_dirty+0x448/0x630 fs/ext4/inode.c:5818
 ext4_dirty_inode+0xa6/0xc0 fs/ext4/inode.c:5854
 __mark_inode_dirty+0x72/0x7f0 fs/fs-writeback.c:2260
 generic_update_time fs/inode.c:1762 [inline]
 update_time fs/inode.c:1775 [inline]
 touch_atime+0x157/0x1a0 fs/inode.c:1846
 file_accessed include/linux/fs.h:2206 [inline]
 iterate_dir+0x31b/0x370 fs/readdir.c:70
 __do_sys_getdents fs/readdir.c:283 [inline]
 __se_sys_getdents+0x7f/0x190 fs/readdir.c:268
 __x64_sys_getdents+0x3f/0x50 fs/readdir.c:268
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812a2b02bc of 4 bytes by task 7 on cpu 0:
 xa_marked include/linux/xarray.h:416 [inline]
 mapping_tagged include/linux/fs.h:493 [inline]
 requeue_inode fs/fs-writeback.c:1409 [inline]
 writeback_sb_inodes+0x958/0xff0 fs/fs-writeback.c:1748
 __writeback_inodes_wb+0xb0/0x2b0 fs/fs-writeback.c:1790
 wb_writeback+0x2b0/0x6f0 fs/fs-writeback.c:1896
 wb_check_start_all fs/fs-writeback.c:2020 [inline]
 wb_do_writeback+0x423/0x5f0 fs/fs-writeback.c:2046
 wb_workfn+0xc8/0x470 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.9.0-rc8-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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