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BUG: KCSAN: data-race in __mnt_want_write_file / propagate_mnt

write to 0xffff8880bae4a030 of 4 bytes by task 18563 on cpu 0:
 propagate_mnt+0x5e8/0x640 fs/pnode.c:330
 attach_recursive_mnt+0x519/0x1210 fs/namespace.c:2129
 graft_tree fs/namespace.c:2237 [inline]
 do_add_mount fs/namespace.c:2784 [inline]
 do_new_mount_fc fs/namespace.c:2823 [inline]
 do_new_mount fs/namespace.c:2877 [inline]
 do_mount+0x1ad5/0x1c00 fs/namespace.c:3200
 __do_sys_mount fs/namespace.c:3410 [inline]
 __se_sys_mount+0xd3/0x100 fs/namespace.c:3387
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3387
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880bae4a030 of 4 bytes by task 18528 on cpu 1:
 __mnt_is_readonly fs/namespace.c:248 [inline]
 mnt_clone_write fs/namespace.c:377 [inline]
 __mnt_want_write_file+0x5c/0x110 fs/namespace.c:398
 file_update_time+0x1c3/0x3f0 fs/inode.c:1987
 file_modified+0x62/0x80 fs/inode.c:2013
 ext4_dio_write_checks fs/ext4/file.c:437 [inline]
 ext4_dio_write_iter fs/ext4/file.c:501 [inline]
 ext4_file_write_iter+0xb84/0x1010 fs/ext4/file.c:653
 call_write_iter include/linux/fs.h:1908 [inline]
 do_iter_readv_writev+0x321/0x3c0 fs/read_write.c:713
 do_iter_write+0x10e/0x470 fs/read_write.c:1018
 vfs_iter_write+0x4c/0x70 fs/read_write.c:1059
 iter_file_splice_write+0x41a/0x770 fs/splice.c:750
 do_splice_from fs/splice.c:846 [inline]
 direct_splice_actor+0x95/0x160 fs/splice.c:1016
 splice_direct_to_actor+0x365/0x660 fs/splice.c:971
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x562/0xb10 fs/read_write.c:1540
 __do_sys_sendfile64 fs/read_write.c:1601 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 18528 Comm: syz-executor.3 Not tainted 5.8.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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