Re: panic with tcpdrop

It is not reproduceable. But a have 2 panic of that in 1 month.

The steps for the last panic:

watch -W /dev/ttyv0
make buildkernel ....
Ctrl-G
Panic!

The explication from Kostik can be correct, but, if I not try to attach 
again, only Ctrl-G; it is not the path for that explication or I 
misunderstands. Anyway, I can probe the changes for 1 month to test if 
any happen
Thanks!
Javier


> On Sat, Nov 24, 2007 at 09:19:42PM +0000, Robert Watson wrote:
>> On Sat, 24 Nov 2007, Rako wrote:
>>
>>> the patch solve the problem with tcpdrop, Thanks!!
>>>
>>> An other panic ocurred, but on other area, is on snp.ko module (watch -W 
>>> /dev/ttyv0) but can't get backtrace. This panic is simliar at
>>>
>>> http://lists.freebsd.org/pipermail/freebsd-current/2007-March/069990.html
>>>
>>> the problem may be at line 164 of /usr/src/sys/dev/snp/snp.c snp = 
>>> ttytosnp(tp);
>>>
>>> where snp get NULL
>>>
>>> but, no familiar with this ... Any idea what can I do to solve the error?
>> I'm having trouble reproducing this -- could you give me a detailed set of 
>> instructions regarding the specific steps I should take to try and get this 
>> panic, if it's reproduceable for you?
>>
>> Thanks,
>>
>> Robert N M Watson
>> Computer Laboratory
>> University of Cambridge
>>
>>> Regards,
>>> Javier
>>>
>>>
>>> Fatal trap 12: page fault while in kernel mode
>>> fault virtual address   = 0x24
>>> fault code              = supervisor read, page not present
>>> instruction pointer     = 0x20:0xc3e4f230
>>> stack pointer           = 0x28:0xd66c3b34
>>> frame pointer           = 0x28:0xd66c3b88
>>> code segment            = base 0x0, limit 0xfffff, type 0x1b
>>>                       = DPL 0, pres 1, def32 1, gran 1
>>> processor eflags        = interrupt enabled, resume, IOPL = 0
>>> current process         = 2216 (make)
>>> trap number             = 12
>>> panic: page fault
>>> KDB: stack backtrace:
>>> db_trace_self_wrapper(c0a5f1ea,d66c39d4,c078878a,c0a5d5f4,c0b5bcc0,...) at 
>>> db_trace_self_wrapper+0x26
>>> kdb_backtrace(c0a5d5f4,c0b5bcc0,c0a1fb8c,d66c39e0,d66c39e0,...) at 
>>> kdb_backtrace+0x29
>>> panic(c0a1fb8c,c0a7c54d,c3e44770,1,1,...) at panic+0xaa
>>> trap_fatal(c3e942b8,0,1,0,c39f5630,...) at trap_fatal+0x303
>>> trap_pfault(0,c39f5630,c39f5630,0,c,...) at trap_pfault+0x250
>>> trap(d66c3af4) at trap+0x382
>>> calltrap() at calltrap+0x6
>>> --- trap 0xc, eip = 0xc3e4f230, esp = 0xd66c3b34, ebp = 0xd66c3b88 ---
>>> snplwrite(c33bf800,d66c3c60,0,d66c3bbc,c0754bec,...) at snplwrite+0x80
>>> ttywrite(c3389600,d66c3c60,0,c39cf5e8,c39f5630,...) at ttywrite+0x39
>>> giant_write(c3389600,d66c3c60,0,0,c0abb080,...) at giant_write+0x6c
>>> devfs_write_f(c39cf5e8,d66c3c60,c3de4800,0,c39f5630,...) at 
>>> devfs_write_f+0x75
>>> dofilewrite(d66c3c60,ffffffff,ffffffff,0,c39cf5e8,...) at dofilewrite+0x97
>>> kern_writev(c39f5630,1,d66c3c60,2813c076,0,...) at kern_writev+0x58
>>> write(c39f5630,d66c3cfc,c,110,c337e630,...) at write+0x4f
>>> syscall(d66c3d38) at syscall+0x335
>>> Xint0x80_syscall() at Xint0x80_syscall+0x20
>>> --- syscall (4, FreeBSD ELF32, write), eip = 0x8083603, esp = 0xbfbfd4ec, 
>>> ebp = 0xbfbfd528 ---
>>> Uptime: 19m14s
>>> Physical memory: 495 MB
>>> Dumping 86 MB: 71 55 39 23 7
> 
> I believe I have a plausible explanation for the panic. Please, look
> at the snpioctl(), SNPSTTY command. First, assume that both the s > 0
> and snoop device has attached tty. Then, snp_tty will be overwritten,
> without detaching the old tty from the snooper. In this case, ttytosnp()
> would not find the snp from tty, returning NULL. This would lead to the
> trace above. This is old kernel bug.
> 
> Now, I shall note that watch(8) does not attach to the new tty without
> detaching from the previous one. But, after destroy_dev_sched() conversion
> have been done for snp(4), actual detach is asynchronous. Since watch(8)
> opens the numbered snpX clone device instead of the master /dev/snp, it
> could reopen the same device. The condition is racy, and thus not easily
> reproducable.
> 
> The patch below might help with kernel panic.
> 
> diff --git a/sys/dev/snp/snp.c b/sys/dev/snp/snp.c
> index a84e90c..b8f3d63 100644
> --- a/sys/dev/snp/snp.c
> +++ b/sys/dev/snp/snp.c
> _at__at_ -491,7 +491,7 _at__at_ snpioctl(struct cdev *dev, u_long cmd, caddr_t data, int flags,
>      struct thread *td)
>  {
>  	struct snoop *snp;
> -	struct tty *tp, *tpo;
> +	struct tty *tp;
>  	struct cdev *tdev;
>  	struct file *fp;
>  	int s;
> _at__at_ -502,6 +502,9 _at__at_ snpioctl(struct cdev *dev, u_long cmd, caddr_t data, int flags,
>  		s = *(int *)data;
>  		if (s < 0)
>  			return (snp_down(snp));
> +		if (snp->snp_tty != NULL)
> +			return (EBUSY);
> +
>  		if (fget(td, s, &fp) != 0)
>  			return (EINVAL);
>  		if (fp->f_type != DTYPE_VNODE ||
> _at__at_ -520,13 +523,6 _at__at_ snpioctl(struct cdev *dev, u_long cmd, caddr_t data, int flags,
>  			return (EBUSY);
>  
>  		s = spltty();
> -
> -		if (snp->snp_target == NULL) {
> -			tpo = snp->snp_tty;
> -			if (tpo)
> -				tpo->t_state &= ~TS_SNOOP;
> -		}
> -
>  		tp->t_state |= TS_SNOOP;
>  		snp->snp_olddisc = tp->t_line;
>  		tp->t_line = snooplinedisc;