Re: panic: mtx_lock_spin: recursed on non-recursive mutex vtbuf @ ..

On Fri, May 22, 2026 at 1:46 AM Bjoern A. Zeeb
<bzeeb-lists@lists.zabbadoz.net> wrote:
>
> Hi,
>
> I was using the mouse in tmux on v1 when everying stopped.
> Sadly we didn't switch to v0 for console but it seems I managed to get a dump only checking now; the kernel from then is already gone.
>
> core.txt said.
>
> panic: mtx_lock_spin: recursed on non-recursive mutex vtbuf @ /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/dev/vt/vt_buf.c:744
>
> cpuid = 1
> time = 1779437567
> KDB: stack backtrace:
> db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe00d773e7a0
> vpanic() at vpanic+0x149/frame 0xfffffe00d773e8d0
> panic() at panic+0x43/frame 0xfffffe00d773e930
> __mtx_lock_spin_flags() at __mtx_lock_spin_flags+0x11b/frame 0xfffffe00d773e970
> vtbuf_flush_mark() at vtbuf_flush_mark+0x82/frame 0xfffffe00d773e9b0
> vtbuf_unmark_on_cross() at vtbuf_unmark_on_cross+0xcc/frame 0xfffffe00d773e9d0
> vtterm_fill() at vtterm_fill+0x27/frame 0xfffffe00d773ea00
> teken_subr_erase_line() at teken_subr_erase_line+0x90/frame 0xfffffe00d773ea20
> teken_state_2() at teken_state_2+0x497/frame 0xfffffe00d773ea40
> teken_input_char() at teken_input_char+0x47/frame 0xfffffe00d773ea60
> teken_input() at teken_input+0x9f/frame 0xfffffe00d773ea90
> termtty_outwakeup() at termtty_outwakeup+0xcf/frame 0xfffffe00d773eb60
> ttydisc_write() at ttydisc_write+0x337/frame 0xfffffe00d773ecd0
> ttydev_write() at ttydev_write+0x13f/frame 0xfffffe00d773ed10
> devfs_write_f() at devfs_write_f+0xf3/frame 0xfffffe00d773ed70
> dofilewrite() at dofilewrite+0x81/frame 0xfffffe00d773edc0
> sys_writev() at sys_writev+0x69/frame 0xfffffe00d773ee00
> amd64_syscall() at amd64_syscall+0x168/frame 0xfffffe00d773ef30
> fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe00d773ef30
> --- syscall (121, FreeBSD ELF64, writev), rip = 0x82601d5aa, rsp = 0x8207a3408, rbp = 0x8207a3430 ---
> KDB: enter: panic
>
>
> In case it helps, I believe this is file:line as reported by gdb in core.txt:
>
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/kern_mutex.c:353
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/dev/vt/vt_buf.c:744
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/dev/vt/vt_buf.c:864
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/dev/vt/vt_buf.c:210
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/dev/vt/vt_buf.c:232
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/dev/vt/vt_core.c:1201
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/teken/teken.c:121
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/teken/teken_subr.h:558
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/teken/teken.c:255
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/teken/teken.c:284
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/teken/teken.c:317
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/subr_terminal.c:422
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/sys/ttydevsw.h:114
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/tty_ttydisc.c:658
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/tty.c:550
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/fs/devfs/devfs_vnops.c:1980
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/sys/file.h:372
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/sys_generic.c:565
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/sys_generic.c:492
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/kern/sys_generic.c:478
>      at /usr/home/test/Sources/git/FreeBSD/freebsd-src.git/sys/amd64/amd64/../../kern/subr_syscall.c:193
>
>
> --
> Bjoern A. Zeeb                                                     r15:7
>

I think it's probably related to or a regression from
8db0553ed6d8 ("vt: Clear cut-paste selection if the area intersects
with the filled region")
https://cgit.freebsd.org/src/commit/?id=8db0553ed6d8636d82a26896237099526b93be19

which added vtbuf_unmark_on_cross() which is in your panic stack.

I don't know this code but it looks like the mutex is first taken by
teken_input / teken_funcs_pre_input / vtterm_pre_input and then again
by teken_input with your panic stack.

Ryan