How best to debug locking/scheduler problems

[Mel Flynn]

On Wednesday 17 June 2009 13:17:37 John Baldwin wrote:
> On Wednesday 17 June 2009 3:52:54 pm Mel Flynn wrote:
> > On Wednesday 17 June 2009 04:15:26 John Baldwin wrote:
> > > On Tuesday 16 June 2009 7:01:45 pm Mel Flynn wrote:
> > > > On Tuesday 16 June 2009 11:02:42 John Baldwin wrote:
> > > > > On Tuesday 16 June 2009 1:52:23 pm Mel Flynn wrote:
> > > > > > Hi John,
> > > > > >
> > > > > > On Tuesday 16 June 2009 04:19:57 John Baldwin wrote:
> > > > > > > On Monday 15 June 2009 5:53:05 pm Mel Flynn wrote:
> > > > > > > >   PID    TID COMM             TDNAME           KSTACK
> > > > > > > >  4283 100215 kdeinit4         -                mi_switch
> > > > > > > > turnstile_wait _mtx_lock_sleep uipc_peeraddr kern_getpeername
> > > > > > > > getpeername syscall Xint0x80_syscall
> > > > > > > > % ps -ww 4283
> > > > > > > >   PID  TT  STAT      TIME COMMAND
> > > > > > > >  4283  ??  T      0:00.38 kdeinit4: kdeinit4: kio_http http
> > > > > > > > local:/tmp/ksocket-mel/klauncherxJ1635.slave-socket
> > > > > > > > local:/tmp/ksocket- mel/plasmayC1653.slave-socket (kdeinit4)
> > > > > > > >
> > > > > > > > %ls -l /tmp/ksocket-mel/
> > > > > > > >
> > > > > > > > total 2
> > > > > > > > -rw-rw-r--  1 mel  wheel  62 Jun 14 22:55 KSMserver__0
> > > > > > > > srw-------  1 mel  wheel   0 Jun 14 22:55 kdeinit4__0
> > > > > > > > srwxrwxr-x  1 mel  wheel   0 Jun 14 22:55
> > > > > > > > klauncherxJ1635.slave-socket
> > > > > > >
> > > > > > > You can use kgdb and the scripts at www.freebsd.org/~jhb/gdb.
> > > > > > > Simply run 'kgdb' as root and do 'lcd /folder/with/scripts' and
> > > > > > > 'source gdb6'. You can then do 'lockchain 4283' to find who
> > > > > > > holds the lock this thread is blocked on and what state they
> > > > > > > are in.
> > > > > >
> > > > > > Looks like a deadlock:
> > > > > >
> > > > > > (kgdb) lockchain 4283
> > > > > >  thread 100215 (pid 4283, kdeinit4) blocked on lock 0xc64374a0
> > > > > > "unp_mtx" thread 100122 (pid 1635, klauncher) blocked on lock
> > > > > > 0xc6806348 "unp_mtx" thread 100215 (pid 4283, kdeinit4) blocked
> > > > > > on lock 0xc64374a0 "unp_mtx" thread 100122 (pid 1635, klauncher)
>
> blocked
>
> > > > > > on lock 0xc6806348 "unp_mtx" thread 100215 (pid 4283, kdeinit4)
> > > > > > blocked on lock 0xc64374a0 "unp_mtx" thread 100122 (pid 1635,
> > > > > > klauncher) blocked on lock 0xc6806348 "unp_mtx" thread 100215
> > > > > > (pid 4283, kdeinit4) blocked on lock 0xc64374a0 "unp_mtx" thread
> > > > > > 100122 (pid 1635, klauncher) blocked on lock 0xc6806348 "unp_mtx"
> > > > > > thread 100215 (pid 4283, kdeinit4) blocked on lock 0xc64374a0
> > > > > > "unp_mtx" thread 100122 (pid 1635, klauncher) blocked on lock
> > > > > > 0xc6806348 "unp_mtx" thread 100215 (pid 4283, kdeinit4) blocked
> > > > > > on lock 0xc64374a0 "unp_mtx" thread 100122 (pid 1635, klauncher)
> > > > > > blocked on lock 0xc6806348 "unp_mtx" thread 100215 (pid 4283,
> > > > > > kdeinit4) blocked on lock 0xc64374a0 "unp_mtx" thread 100122 (pid
> > > > > > 1635, klauncher) blocked on lock 0xc6806348 "unp_mtx" thread
> > > > > > 100215 (pid 4283, kdeinit4) blocked on lock 0xc64374a0 "unp_mtx"
> > > > > > thread 100122 (pid 1635, klauncher) blocked on lock 0xc6806348
> > > > > > "unp_mtx" thread 100215 (pid 4283, kdeinit4) blocked on lock
> > > > > > 0xc64374a0 "unp_mtx" thread 100122 (pid 1635, klauncher) blocked
> > > > > > on lock 0xc6806348 "unp_mtx" thread 100215 (pid 4283, kdeinit4)
> > > > > > blocked on lock 0xc64374a0 "unp_mtx" thread 100122 (pid 1635,
> > > > > > klauncher) blocked on lock 0xc6806348 "unp_mtx" DEADLOCK
> > > > > >
> > > > > > Looking through the scripts now to see how I can get more info on
>
> the
>
> > > > > > call chain and hoping I don't panic the machine ;). It is quite
> > > > > > random to reproduce.
> > > > >
> > > > > In kgdb you can simply do 'tid 100122' followed by 'bt' and 'tid
> > > > > 100215' followed by 'bt'.
> > > >
> > > > Cool, thanks for helping John. Of course it pretty much shows me what
> > > > procstat -k shows and can't get any info on the userland part, but I
> > > > can fully inspect the locks and threads.
> > > >
> > > > Both threads are in TDS_INHIBITED state, and blocked on:
> > > > (kgdb) frame 0
> > > > #0  sched_switch (td=0xc5971240, newtd=0xc4d39900, flags=259)
> > > >     at /usr/src/sys/kern/sched_ule.c:1864
> > > > 1864                    cpuid = PCPU_GET(cpuid);
> > >
> > > That doesn't really tell us anything except that it isn't running.  We
>
> know
>
> > > it is actually blocked on a lock, and we need the full stack trace to
> > > see where the two threads were trying to acquire the locks, hence 'bt'.
> > >  ' procstat -k' output would be fine, too.
> >
> > the respective bt's:
> > (kgdb) tid 100122
> >     at /usr/src/sys/kern/kern_mutex.c:447
> > #4  0xc06a68a5 in uipc_peeraddr (so=0xc64309a8, nam=0xe79a2c70)
> >     at /usr/src/sys/kern/uipc_usrreq.c:682
> > #5  0xc06a1e71 in kern_getpeername (td=0xc56e8900, fd=12, sa=0xe79a2c70,
> > alen=0xe79a2c6c)
> >     at /usr/src/sys/kern/uipc_syscalls.c:1566
> >
> > (kgdb) tid 100215
> > (kgdb) bt
> >     at /usr/src/sys/kern/kern_mutex.c:447
> > #4  0xc06a68a5 in uipc_peeraddr (so=0xc6976338, nam=0xe9ae9c70)
> >     at /usr/src/sys/kern/uipc_usrreq.c:682
> > #5  0xc06a1e71 in kern_getpeername (td=0xc5971240, fd=7, sa=0xe9ae9c70,
> > alen=0xe9ae9c6c)
> >     at /usr/src/sys/kern/uipc_syscalls.c:1566
>
> These are the key frames.  It looks like uipc_peeraddr() tries to lock two
> unp locks w/o any protection from the global unp linkage lock.  I've
> changed it to use the same locking as uipc_accept() where it first grabs a
> read lock on the linkage lock and then just locks the other end of the
> connection to copy out its sockaddr.

Thanks John. I'll recompile the kernel with patch and up-to-date current and 
report back if there are any side effects or if the bug resurfaces.
Is there a sure way (i.e. testcase) that would expose this condition? At 
present, all I can do is wait and maybe play with network interface link 
up/down, as it seems to be related from a high level view.

-- 
Mel