"panic: mutex Giant not owned" in do_execve()

quad e450 running up-to-date -current:

panic: mutex Giant not owned at ../../../kern/kern_mutex.c:299
cpuid = 0
KDB: enter: panic
[thread pid 52851 tid 100456 ]
Stopped at      kdb_enter+0x3c: ta              %xcc, 1
db> wh
Tracing pid 52851 tid 100456 td 0xfffff80077c61560
panic() at panic+0x16c
_mtx_assert() at _mtx_assert+0x6c
_mtx_unlock_flags() at _mtx_unlock_flags+0x68
do_execve() at do_execve+0xa0c
kern_execve() at kern_execve+0x7c
execve() at execve+0x38
syscall() at syscall+0x2d4
-- reserved %o7=0 --
userland() at 0x40223400
user trace: trap %o7=0
pc 0x40223400, sp 0x7fdffffd021
done

#12 0x00000000c01525cc in do_execve (td=0xfffff80077c61560, args=0xc, mac_p=0x0) at ../../../kern/kern_exec.c:789
#13 0x00000000c0151b3c in kern_execve (td=0xfffff80077c61560, args=0xeea6f670, mac_p=0x0) at ../../../kern/kern_exec.c:250
#14 0x00000000c0151a78 in execve (td=0xfffff80077c61560, uap=0xeea6f8c0) at ../../../kern/kern_exec.c:185
#15 0x00000000c02f3cd4 in syscall (tf=0xeea6f880) at ../../../sparc64/sparc64/trap.c:592
(kgdb) frame 12
#12 0x00000000c01525cc in do_execve (td=0xfffff80077c61560, args=0xc, mac_p=0x0) at ../../../kern/kern_exec.c:789
789             VFS_UNLOCK_GIANT(vfslocked);
(kgdb) list
784     #ifdef MAC
785             mac_execve_exit(imgp);
786             if (interplabel != NULL)
787                     mac_vnode_label_free(interplabel);
788     #endif
789             VFS_UNLOCK_GIANT(vfslocked);
790             return (error);
791     }
792
793     int
(kgdb) frame 13
#13 0x00000000c0151b3c in kern_execve (td=0xfffff80077c61560, args=0xeea6f670, mac_p=0x0) at ../../../kern/kern_exec.c:250
250             error = do_execve(td, args, mac_p);
(kgdb) list
245                             return (ERESTART);      /* Try again later. */
246                     }
247                     PROC_UNLOCK(p);
248             }
249
250             error = do_execve(td, args, mac_p);
251
252             if (p->p_flag & P_HADTHREADS) {
253                     PROC_LOCK(p);
254                     /*
(kgdb) frame 14
#14 0x00000000c0151a78 in execve (td=0xfffff80077c61560, uap=0xeea6f8c0) at ../../../kern/kern_exec.c:185
185                     error = kern_execve(td, &args, NULL);

Kris