exclusive sleep mutex mntvnode r = 0 (0xffffffff80758220) locked _at_ /a/asami/portbuild/amd64/src-client/sys/ufs/ffs/ffs_vfsops.c:1172

One of the amd64 machines died with the following.  The kernel is a
few weeks old, so this might already be fixed.

Kris

malloc() of "64" with the following non-sleepable locks held:
exclusive sleep mutex mntvnode r = 0 (0xffffffff80758220) locked _at_ /a/asami/portbuild/amd64/src-client/sys/ufs/ffs/ffs_vfsops.c:1172
recursed on non-recursive lock (sleep mutex) mntvnode _at_ /a/asami/portbuild/amd64/src-client/sys/kern/vfs_subr.c:1054
first acquired _at_ /a/asami/portbuild/amd64/src-client/sys/ufs/ffs/ffs_vfsops.c:1172
panic: recurse
Debugger("panic")
Stopped at      Debugger+0x4b:  xchgl   %ebx,0x31599f
db> where
Debugger() at Debugger+0x4b
panic() at panic+0x169
witness_lock() at witness_lock+0x383
_mtx_lock_flags() at _mtx_lock_flags+0x9c
insmntque() at insmntque+0x2a
vclean() at vclean+0x35b
vgonel() at vgonel+0x51
vrecycle() at vrecycle+0x5b
ufs_inactive() at ufs_inactive+0x22c
ufs_vnoperate() at ufs_vnoperate+0x14
vrele() at vrele+0x11a
ffs_sync() at ffs_sync+0x24f
sync() at sync+0xdb
syscall() at syscall+0x320
Xfast_syscall() at Xfast_syscall+0xa7
--- syscall (36, FreeBSD ELF64, sync), rip = 0x402084, rsp = 0x7ffffffff648, rbp = 0x3 ---
db>