Re: panic in deadlkres

On 25 June 2010 13:50, Anton Yuzhaninov <citrin_at_citrin.ru> wrote:
> I've got panic on 9-current from Jun 25 2010
>
> May be this is bug in deadlock resolver
>
> panic: blockable sleep lock (sleep mutex) process lock _at_
> /usr/src/sys/kern/kern_clock.c:203
>
> db> show alllocks
> Process 0 (kernel) thread 0xc4dcd270 (100047)
> shared sx allproc (allproc) r = 0 (0xc0885ebc) locked _at_
> /usr/src/sys/kern/kern_clock.c:193
>
> db> show lock 0xc4dcd270
>  class: spin mutex
>  name: D
>  flags: {SPIN, RECURSE}
>  state: {OWNED}
>
> (kgdb) bt
> #0  doadump () at pcpu.h:248
> #1  0xc05ae59f in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:416
> #2  0xc05ae825 in panic (fmt=Variable "fmt" is not available.
> ) at /usr/src/sys/kern/kern_shutdown.c:590
> #3  0xc048ff45 in db_panic (addr=Could not find the frame base for "db_panic".
> ) at /usr/src/sys/ddb/db_command.c:478
> #4  0xc0490533 in db_command (last_cmdp=0xc086ef1c, cmd_table=0x0, dopager=1) at /usr/src/sys/ddb/db_command.c:445
> #5  0xc0490662 in db_command_loop () at /usr/src/sys/ddb/db_command.c:498
> #6  0xc04923ef in db_trap (type=3, code=0) at /usr/src/sys/ddb/db_main.c:229
> #7  0xc05dade6 in kdb_trap (type=3, code=0, tf=0xc4b31bd0) at /usr/src/sys/kern/subr_kdb.c:535
> #8  0xc078696b in trap (frame=0xc4b31bd0) at /usr/src/sys/i386/i386/trap.c:692
> #9  0xc076ca0b in calltrap () at /usr/src/sys/i386/i386/exception.s:165
> #10 0xc05daf30 in kdb_enter (why=0xc07ea02d "panic", msg=0xc07ea02d "panic") at cpufunc.h:71
> #11 0xc05ae806 in panic (fmt=0xc07efd94 "blockable sleep lock (%s) %s _at_ %s:%d") at /usr/src/sys/kern/kern_shutdown.c:573
> #12 0xc05ee30b in witness_checkorder (lock=0xc5148088, flags=9, file=0xc07e3b20 "/usr/src/sys/kern/kern_clock.c", line=203, interlock=0x0)
>    at /usr/src/sys/kern/subr_witness.c:1067
> #13 0xc05a093c in _mtx_lock_flags (m=0xc5148088, opts=0, file=0xc07e3b20 "/usr/src/sys/kern/kern_clock.c", line=203)
>    at /usr/src/sys/kern/kern_mutex.c:200
> #14 0xc05706a9 in deadlkres () at /usr/src/sys/kern/kern_clock.c:203
> #15 0xc0588721 in fork_exit (callout=0xc05705ea <deadlkres>, arg=0x0, frame=0xc4b31d38) at /usr/src/sys/kern/kern_fork.c:843
> #16 0xc076ca80 in fork_trampoline () at /usr/src/sys/i386/i386/exception.s:270

Hi!

[throw in ideas (just ignore them if they're dumb, thinking badly atm).]

AFAIK, that indicates that some thread already has
a spin mutex and then it tries to acquire a sleep mutex.

Looks like kern/kern_clock.c v1.213 (SVN rev 206482)
has a regression in handling ticks wrap-up
w.r.t. it doesn't release a thread mutex, does it?

>From subr_witness.c:
1062:                 * Since spin locks include a critical section, this check
1063:                 * implicitly enforces a lock order of all sleep
locks before
1064:                 * all spin locks.
1065:                 */
1066:                if (td->td_critnest != 0 && !kdb_active)
1067:                        panic("blockable sleep lock (%s) %s _at_ %s:%d",
1068:                            class->lc_name, lock->lo_name, file, line);

>From kern_clock.c, v1.213 (in several places, while holding a thread lock):
+					/* Handle ticks wrap-up. */
+					if (ticks < td->td_blktick)
+						continue;

Should not it be like the next:
+					/* Handle ticks wrap-up. */
+					if (ticks < td->td_blktick) {
+						thread_unlock(td);
+						continue;
+					}

The precondition idea to reproduce it is to lock a subject thread
in some deadlkres callout, handle re-wrap condition, then try
to lock a process to witch the thread belongs in (n+m)'th deadlkres
callout, or in different context.

-- 
wbr,
pluknet