panic: knlist locked, but should not be

Hi,

I'm hitting this "knlist locked"-panic when using BPF descriptors 
together with kqueue/kevent.

panic: knlist locked, but should not be

#10 0xc051a1e3 in knlist_add (knl=0xc3b69460, kn=0xc3ad65d8, islocked=0)
     at /usr/src/sys/kern/kern_event.c:1571
#11 0xc059c5de in bpfkqfilter (dev=0x12, kn=0xc3ad65d8) at 
/usr/src/sys/net/bpf.c:1194
#12 0xc050d9e6 in giant_kqfilter (dev=0xc38f9d00, kn=0xc3ad65d8)
     at /usr/src/sys/kern/kern_conf.c:336
#13 0xc04e9580 in devfs_kqfilter_f (fp=0xc3ad40d8, kn=0xc3ad65d8)
     at /usr/src/sys/fs/devfs/devfs_vnops.c:444
#14 0xc0517e6d in filt_fileattach (kn=0x12) at file.h:305
#15 0xc0518eb0 in kqueue_register (kq=0xc3923780, kev=0xe3a67b9c, 
td=0xc3714a20, waitok=1)
     at /usr/src/sys/kern/kern_event.c:902
#16 0xc05185f4 in kern_kevent (td=0xc3714a20, fd=3, nchanges=1, 
nevents=0, k_ops=0xe3a67c70,
     timeout=0x0) at /usr/src/sys/kern/kern_event.c:640
#17 0xc0518491 in kevent (td=0xc3714a20, uap=0xe3a67d04) at 
/usr/src/sys/kern/kern_event.c:572

I thinks the problem is as follows, islocked is 0 which triggers
the macro KNL_ASSERT_LOCK(knl, islocked);
(It will only bite you if you run with INVARIANTS on)

The knlist is initialized with the same mutex that protects the
rest of the bpf_d structure.
bpfopen(), sys/net/bpf.c:383

	knlist_init(&d->bd_sel.si_note, &d->bd_mtx, NULL, NULL, NULL);

In bpfkqfilter() and filt_bpfdetach() this mutex is acquired with
BPFD_LOCK() before calling knlist_add()/knlist_remove()

	#define BPFD_LOCK(bd)           mtx_lock(&(bd)->bd_mtx)

Hence, the knlist lock IS locked despite being called with islocked = 0.
If using the same mutex doesn't cause any additional problems,
(I haven't noticed anything yet, but there could of course be issues
that I've overlooked), I think knlist_add should be called
with islocked = 1. A suggested patch is attached.


Fredrik Lindberg

Index: bpf.c
===================================================================
RCS file: /home/ncvs/src/sys/net/bpf.c,v
retrieving revision 1.168
diff -u -r1.168 bpf.c
--- bpf.c	15 Jun 2006 15:39:12 -0000	1.168
+++ bpf.c	3 Jul 2006 11:36:30 -0000
_at__at_ -1152,7 +1152,7 _at__at_
 	d->bd_pid = curthread->td_proc->p_pid;
 	kn->kn_fop = &bpfread_filtops;
 	kn->kn_hook = d;
-	knlist_add(&d->bd_sel.si_note, kn, 0);
+	knlist_add(&d->bd_sel.si_note, kn, 1);
 	BPFD_UNLOCK(d);
 
 	return (0);
_at__at_ -1164,7 +1164,7 _at__at_
 	struct bpf_d *d = (struct bpf_d *)kn->kn_hook;
 
 	BPFD_LOCK(d);
-	knlist_remove(&d->bd_sel.si_note, kn, 0);
+	knlist_remove(&d->bd_sel.si_note, kn, 1);
 	BPFD_UNLOCK(d);
 }