Re: new panic in cpu_reset() with WITNESS

on 24/01/2012 14:32 Gleb Smirnoff said the following:
> Yes, now:
> 
> Rebooting...
> lock order reversal:
>  1st 0xffffffff80937140 smp rendezvous (smp rendezvous) _at_ /usr/src/head/sys/kern/kern_shutdown.c:542
>  2nd 0xfffffe0001f5d838 uart_hwmtx (uart_hwmtx) _at_ /usr/src/head/sys/dev/uart/uart_cpu.h:92
> panic: mtx_lock_spin: recursed on non-recursive mutex cnputs_mtx _at_ /usr/src/head/sys/kern/kern_cons.c:500

OK, so it's just a plain LOR between smp rendezvous and uart_hwmtx, no new
details...

It's still intriguing to me why the LOR *doesn't* happen [*] with
stop_scheduler_on_panic=0.  But I don't see a productive way to pursue this
investigation further.

[*] - or maybe better to say why it isn't detected/reported.

> cpuid = 0
> KDB: enter: panic
> [ thread pid 1 tid 100001 ]
> Stopped at      kdb_enter+0x3b: movq    $0,0x5159f2(%rip)
> db> bt
> Tracing pid 1 tid 100001 td 0xfffffe0001d5e000
> kdb_enter() at kdb_enter+0x3b
> panic() at panic+0x1c7
> _mtx_lock_spin_flags() at _mtx_lock_spin_flags+0x10f
> cnputs() at cnputs+0x7a
> putchar() at putchar+0x11f
> kvprintf() at kvprintf+0x83
> vprintf() at vprintf+0x85
> printf() at printf+0x67

Maybe kdb_backtrace ought to use db_printf.

> kdb_backtrace() at kdb_backtrace+0x2d
> _witness_debugger() at _witness_debugger+0x2c
> witness_checkorder() at witness_checkorder+0x854
> _mtx_lock_spin_flags() at _mtx_lock_spin_flags+0x99
> uart_cnputc() at uart_cnputc+0x3e
> cnputc() at cnputc+0x4c
> cnputs() at cnputs+0x26
> putchar() at putchar+0x11f
> kvprintf() at kvprintf+0x83
> vprintf() at vprintf+0x85
> printf() at printf+0x67
> cpu_reset() at cpu_reset+0x81
> kern_reboot() at kern_reboot+0x3a5
> sys_reboot() at sys_reboot+0x42
> amd64_syscall() at amd64_syscall+0x39e
> Xfast_syscall() at Xfast_syscall+0xf7
> --- syscall (55, FreeBSD ELF64, sys_reboot), rip = 0x40ea3c, rsp = 0x7fffffffd6d8, rbp = 0x49 ---

-- 
Andriy Gapon