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BUG: KCSAN: data-race in __mnt_want_write_file / propagate_one

write to 0xffff88812a99e970 of 4 bytes by task 19698 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_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 0xffff88812a99e970 of 4 bytes by task 19695 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
 ext4_page_mkwrite+0xe3/0xae0 fs/ext4/inode.c:5972
 do_page_mkwrite mm/memory.c:2527 [inline]
 do_shared_fault+0x21e/0x770 mm/memory.c:3921
 do_fault mm/memory.c:3989 [inline]
 handle_pte_fault mm/memory.c:4225 [inline]
 __handle_mm_fault mm/memory.c:4357 [inline]
 handle_mm_fault+0x158a/0x1950 mm/memory.c:4394
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1295
 handle_page_fault arch/x86/mm/fault.c:1365 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1418
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:549

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 19695 Comm: syz-executor.5 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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