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BUG: KCSAN: data-race in __mnt_want_write_file / propagate_one

write to 0xffff88812a885970 of 4 bytes by task 1015 on cpu 0:
 propagate_one+0x32d/0x400 fs/pnode.c:267
 propagate_mnt+0x2f8/0x640 fs/pnode.c:319
 attach_recursive_mnt+0x519/0x1210 fs/namespace.c:2129
 graft_tree fs/namespace.c:2237 [inline]
 do_loopback fs/namespace.c:2365 [inline]
 do_mount+0x11ed/0x1bd0 fs/namespace.c:3193
 __do_sys_mount fs/namespace.c:3409 [inline]
 __se_sys_mount+0xd3/0x100 fs/namespace.c:3386
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3386
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812a885970 of 4 bytes by task 8273 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_write_checks fs/ext4/file.c:249 [inline]
 ext4_buffered_write_iter+0x1c8/0x390 fs/ext4/file.c:265
 ext4_file_write_iter+0x4a3/0xfd0 fs/ext4/file.c:498
 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+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8273 Comm: rs:main Q:Reg Not tainted 5.8.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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