Re: Occasional wedging entering DDB via serial break

On Sun, Dec 26, 2004 at 10:37:32PM +0000, Robert Watson wrote:
> 
> I'm seeing occasional wedging attempting to enter DDB via a serial break
> on a dual-Xeon box (4 logical processors).  The symptoms usually look
> something like this:
> 
> hippy# ./tmp.csh
> ~KDB: enter: Line break on console
> [thread pid 560 tid 100202 ]
> Stopped at      kdb_enter+0x2c: leave
> db> show alllocks
> db> cont
> load: 0.04  cmd: super-smack 619 [runnable] 0.02u 0.51s 2% 2216k
> ~KDB: enter: Line break on console
> [thread pid 560 tid 100201 ]
> Stopped at      kdb_enter+0x2c: leave
> db> show alllocks
> Process 560 (mysqld)
> Thread 0x18769
> exclusive sleep mutex so_rcv r = 0 (0xc2a25bcc) locked _at_
> kern/uipc_usrreq.c:464
> exclusive sleep mutex unp r = 0 (0xc091a740) locked _at_
> kern/uipc_usrreq.c:392
> db> cont
> load: 0.92  cmd: super-smack 619 [runnable] 0.05u 1.36s 6% 2216k
> ~KDB: enter: Line break on console
> [thread pid 560 tid 100199 ]
> Stopped at      kdb_enter+0x2c: leave
> db> show alllocks
> db> cont
> load: 2.12  cmd: super-smack 616 [running] 0.08u 2.90s 10% 2216k
> ~KDB: enter: Line break on console
> <wedge>
> 
> After that point, a serial break will no longer drop to DDB, respond to
> pings, etc.  The box is running 6.x-CURRENT from this morning.  Is anyone
> else seeing this, or does anyone else have ideas about what might cause
> this?

I see much the same..it seems to be just one more way in which DDB is
broken on SMP machines.  Setting debug.kdb.stop_cpus=0 tends to fix
this for me, but then I get the joy of overlapping panics from
multiple CPUs.

Kris