I got two off-list mails suggesting LOCKDEBUG. I tried that today - I added LOCKDEBUG and VNODE_LOCKDEBUG both (the latter I found in the ALL config when grepping for LOCKDEBUG).
That kernel panicked promptly on boot panic: vop_read: vp: locked 0, expected 1 with a stack trace that goes breakpoint, panic, VOP_READ, vn_rdwr, check_exec, execve1, sys_execve, start_init. I haven't yet dug any deeper; I don't know whether this is part of my problem (probably not, I would guess, because of the apparent puffs connection), a bug I introduced and just never noticed before, or something broken with one (or both!) of those options. I'll be investigating to figure out which of those it is, but, for the moment, LOCKDEBUG is off the table. :( I'm also going to be using a different machine for at least some of my testing; rebooting this machine this much is...inconvenient. /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mo...@rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B