mtrash_ctor() on standard malloc zones also panics if it detects use after free unconditionally. So may be its good to keep both in sync instead of a panic knob with sysctl/options.

We uncovered/realized this bug and others only after we committed this patch. So I think its really a useful check to have in .
https://svnweb.freebsd.org/base?view=revision&revision=281599

We have not seen any side effect other than this mutex issue for a month+ invariant build testing. I am thinking its good to go now.

The mtx init code is assuming the object being initialized with zeros. Please make this change too in v_addpollinfo() in
freebsd/sys/kern/vfs_subr.c. Otherwise it panics because of uma_junk(0xdeadc0de) in the newly created object out of this zone.