[moderation] [serial?] KCSAN: data-race in __stop_tty / __uart_start

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a0c83177734a Merge tag 'drm-fixes-2026-03-21' of https://g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14aa0b52580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
dashboard link: https://syzkaller.appspot.com/bug?extid=e5f02961146b84f62fae
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [gre...@linuxfoundation.org jiri...@kernel.org linux-...@vger.kernel.org linux-...@vger.kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/41a0e5c919b7/disk-a0c83177.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/70408718214e/vmlinux-a0c83177.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b610cc9ea81f/bzImage-a0c83177.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e5f029...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __stop_tty / __uart_start

write to 0xffff88813277f1bc of 1 bytes by task 32434 on cpu 1:
__stop_tty+0x36/0x90 drivers/tty/tty_io.c:744
n_tty_ioctl_helper+0x2d1/0x370 drivers/tty/tty_ioctl.c:951
n_tty_ioctl+0x101/0x230 drivers/tty/n_tty.c:2496
tty_ioctl+0x83e/0xb80 drivers/tty/tty_io.c:2801
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
__x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
x64_sys_call+0x1563/0x3020 arch/x86/include/generated/asm/syscalls_64.h:17
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88813277f1bc of 1 bytes by task 32428 on cpu 0:
uart_tx_stopped include/linux/serial_core.h:1149 [inline]
__uart_start+0x8a/0x2d0 drivers/tty/serial/serial_core.c:143
uart_write+0x186/0x530 drivers/tty/serial/serial_core.c:633
process_output_block drivers/tty/n_tty.c:557 [inline]
n_tty_write+0x856/0xc20 drivers/tty/n_tty.c:2366
iterate_tty_write drivers/tty/tty_io.c:1006 [inline]
file_tty_write+0x378/0x690 drivers/tty/tty_io.c:1081
tty_write+0x25/0x30 drivers/tty/tty_io.c:1102
iter_file_splice_write+0x6c4/0xa80 fs/splice.c:736
do_splice_from fs/splice.c:936 [inline]
direct_splice_actor+0x156/0x2a0 fs/splice.c:1159
splice_direct_to_actor+0x311/0x670 fs/splice.c:1103
do_splice_direct_actor fs/splice.c:1202 [inline]
do_splice_direct+0x119/0x1a0 fs/splice.c:1228
do_sendfile+0x382/0x650 fs/read_write.c:1372
__do_sys_sendfile64 fs/read_write.c:1433 [inline]
__se_sys_sendfile64 fs/read_write.c:1419 [inline]
__x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1419
x64_sys_call+0x2dc4/0x3020 arch/x86/include/generated/asm/syscalls_64.h:41
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 32428 Comm: syz.8.23941 Tainted: G W syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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