[moderation] [fs?] KCSAN: data-race in __lookup_mnt / __umount_mnt

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f105f3631d51 Merge tag 'x86-urgent-2026-07-05' of git://gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11321b39580000
kernel config: https://syzkaller.appspot.com/x/.config?x=84b3039e8461eef5
dashboard link: https://syzkaller.appspot.com/bug?extid=37046119e4da20fabeb1
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
CC: [bra...@kernel.org ja...@suse.cz linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4f431f8e6ae2/disk-f105f363.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6fa69bae0583/vmlinux-f105f363.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5e6879481290/bzImage-f105f363.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+370461...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __lookup_mnt / __umount_mnt

write to 0xffff888104f6aa90 of 8 bytes by task 13876 on cpu 0:
__umount_mnt+0x4e/0x2c0 fs/namespace.c:994
umount_mnt fs/namespace.c:1009 [inline]
umount_tree+0x5ff/0x8a0 fs/namespace.c:1824
do_umount fs/namespace.c:-1 [inline]
path_umount+0x799/0x7f0 fs/namespace.c:2047
ksys_umount fs/namespace.c:2070 [inline]
__do_sys_umount fs/namespace.c:2075 [inline]
__se_sys_umount fs/namespace.c:2073 [inline]
__x64_sys_umount+0xb6/0xe0 fs/namespace.c:2073
x64_sys_call+0x103b/0x3020 arch/x86/include/generated/asm/syscalls_64.h:167
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888104f6aa90 of 8 bytes by task 2980 on cpu 1:
__lookup_mnt+0x84/0xf0 fs/namespace.c:796
__follow_mount_rcu+0xbc/0x2a0 fs/namei.c:1708
handle_mounts fs/namei.c:1740 [inline]
step_into_slowpath+0x30c/0x4c0 fs/namei.c:2104
step_into fs/namei.c:2152 [inline]
walk_component fs/namei.c:2288 [inline]
link_path_walk+0x947/0xdd0 fs/namei.c:-1
path_lookupat+0x53/0x4c0 fs/namei.c:2812
filename_lookup+0x18f/0x390 fs/namei.c:2842
do_readlinkat+0x74/0x2f0 fs/stat.c:571
__do_sys_readlink fs/stat.c:605 [inline]
__se_sys_readlink fs/stat.c:602 [inline]
__x64_sys_readlink+0x47/0x60 fs/stat.c:602
x64_sys_call+0x2b51/0x3020 arch/x86/include/generated/asm/syscalls_64.h:90
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888119982780 -> 0xffff888104f6aa80

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 2980 Comm: udevd Tainted: G W syzkaller #0 PREEMPT(lazy)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================


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