Re: amd64 head -r329465 (non-debug build, but with symbols): "panic: spin lock held too long" during make check-old, reported during a sys_vfork

> I committed the fix in
> https://svnweb.freebsd.org/base?view=revision&revision=329542
>
> i.e. should be stable from this point on.

Hi!

It is maybe unrelated, but recent commits have broken my system with a 
similar error. I did not have panics with a system built around 
December, but since updating first to r329555 then today to r329641 I’m 
getting a reproducible panic when logging out from a Lumina desktop session:

Unread portion of the kernel message buffer:
spin lock 0xfffff8000d440020 (process slock) held by 0xfffff8000daed560 
(tid 100111) too long
panic: spin lock held too long
cpuid = 1
time = 1519143505
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 
0xfffffe00005c15e0
vpanic() at vpanic+0x18d/frame 0xfffffe00005c1640
panic() at panic+0x43/frame 0xfffffe00005c16a0
_mtx_lock_indefinite_check() at _mtx_lock_indefinite_check+0x71/frame 
0xfffffe00005c16b0
mtx_spin_wait_unlocked() at mtx_spin_wait_unlocked+0x59/frame 
0xfffffe00005c16e0
proc_reap() at proc_reap+0x24/frame 0xfffffe00005c1720
procdesc_close() at procdesc_close+0x125/frame 0xfffffe00005c1760
closef() at closef+0x251/frame 0xfffffe00005c17f0
fdescfree_fds() at fdescfree_fds+0x90/frame 0xfffffe00005c1840
fdescfree() at fdescfree+0x4df/frame 0xfffffe00005c1900
exit1() at exit1+0x508/frame 0xfffffe00005c1970
sys_sys_exit() at sys_sys_exit+0xd/frame 0xfffffe00005c1980
amd64_syscall() at amd64_syscall+0xa48/frame 0xfffffe00005c1ab0
fast_syscall_common() at fast_syscall_common+0x101/frame 0x7fffffffea90
Uptime: 17m45s
Dumping 327 out of 3990 MB:..5%..15%..25%..35%..44%..54%..64%..74%..84%..93%

Reading symbols from /boot/kernel/linux.ko...done.
Loaded symbols for /boot/kernel/linux.ko
Reading symbols from /boot/kernel/linux_common.ko...done.
Loaded symbols for /boot/kernel/linux_common.ko
Reading symbols from /boot/kernel/acpi_ibm.ko...done.
Loaded symbols for /boot/kernel/acpi_ibm.ko
Reading symbols from /boot/kernel/iwm7260fw.ko...done.
Loaded symbols for /boot/kernel/iwm7260fw.ko
Reading symbols from /boot/kernel/coretemp.ko...done.
Loaded symbols for /boot/kernel/coretemp.ko
Reading symbols from /boot/kernel/if_iwm.ko...done.
Loaded symbols for /boot/kernel/if_iwm.ko
Reading symbols from /boot/kernel/acpi_video.ko...done.
Loaded symbols for /boot/kernel/acpi_video.ko
Reading symbols from /boot/kernel/nullfs.ko...done.
Loaded symbols for /boot/kernel/nullfs.ko
Reading symbols from /boot/kernel/fdescfs.ko...done.
Loaded symbols for /boot/kernel/fdescfs.ko
Reading symbols from /boot/kernel/i915kms.ko...done.
Loaded symbols for /boot/kernel/i915kms.ko
Reading symbols from /boot/kernel/drm2.ko...done.
Loaded symbols for /boot/kernel/drm2.ko
Reading symbols from /boot/kernel/iicbus.ko...done.
Loaded symbols for /boot/kernel/iicbus.ko
Reading symbols from /boot/kernel/iic.ko...done.
Loaded symbols for /boot/kernel/iic.ko
Reading symbols from /boot/kernel/iicbb.ko...done.
Loaded symbols for /boot/kernel/iicbb.ko
#0  cpustop_handler () at /usr/src/sys/x86/x86/mp_x86.c:1324
1324            CPU_SET_ATOMIC(cpu, &stopped_cpus);
(kgdb) bt
#0  cpustop_handler () at /usr/src/sys/x86/x86/mp_x86.c:1324
#1  0xffffffff80e29fb4 in ipi_nmi_handler () at 
/usr/src/sys/x86/x86/mp_x86.c:1280
#2  0xffffffff80d09a79 in trap (frame=0xffffffff8158bef0)
     at /usr/src/sys/amd64/amd64/trap.c:188
#3  0xffffffff80cec054 in nmi_calltrap () at 
/usr/src/sys/amd64/amd64/exception.S:633
#4  0xffffffff80e1aaef in acpi_cpu_idle_mwait (mwait_hint=0) at 
cpufunc.h:611
Previous frame inner to this frame (corrupt stack?)
Current language:  auto; currently minimal

kgdb is over my head, but I can provide more details under some guidance.

Hope it helps,
Juan