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BUG: KCSAN: data-race in mounts_open_common / umount_tree

write to 0xffff88821b4406e8 of 8 bytes by task 9092 on cpu 1:
 __touch_mnt_namespace fs/namespace.c:796 [inline]
 umount_tree+0x532/0x870 fs/namespace.c:1459
 do_umount fs/namespace.c:1585 [inline]
 ksys_umount+0x528/0xaf0 fs/namespace.c:1698
 __do_sys_umount fs/namespace.c:1709 [inline]
 __se_sys_umount fs/namespace.c:1707 [inline]
 __x64_sys_umount+0x35/0x50 fs/namespace.c:1707
 do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88821b4406e8 of 8 bytes by task 1 on cpu 0:
 mounts_open_common+0x1d8/0x3e0 fs/proc_namespace.c:276
 mountinfo_open+0x2a/0x40 fs/proc_namespace.c:310
 do_dentry_open+0x306/0x970 fs/open.c:797
 vfs_open+0x62/0x80 fs/open.c:911
 do_open fs/namei.c:3229 [inline]
 path_openat+0x1ff4/0x23d0 fs/namei.c:3346
 do_filp_open+0x11e/0x1b0 fs/namei.c:3373
 do_sys_openat2+0x52e/0x680 fs/open.c:1148
 do_sys_open+0xa2/0x110 fs/open.c:1164
 ksys_open include/linux/syscalls.h:1386 [inline]
 __do_sys_open fs/open.c:1170 [inline]
 __se_sys_open fs/open.c:1168 [inline]
 __x64_sys_open+0x50/0x60 fs/open.c:1168
 do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 1 Comm: systemd Not tainted 5.7.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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