strace, holding sigacts lock over postsig(), et al.

Got a bug report this evening that the strace package hangs on 5-CURRENT.
I'm able to confirm this; for those that don't know, strace makes
extensive use of procfs.  On attempting to reproduce it, I first got:

crash2# strace ls
Sleeping on "stopevent" with the following non-sleepable locks held:
exclusive sleep mutex sigacts r = 0 (0xc20e2aa8) locked _at_
kern/subr_trap.c:260
lock order reversal
 1st 0xc20e2aa8 sigacts (sigacts) _at_ kern/subr_trap.c:260
 2nd 0xc20f1224 process lock (process lock) _at_ kern/kern_synch.c:309
Stack backtrace:
backtrace(c0864c4a,c20f1224,c0860e7b,c0860e7b,c0861ee5) at backtrace+0x17
witness_lock(c20f1224,8,c0861ee5,135,c20f1224) at witness_lock+0x672
_mtx_lock_flags(c20f1224,0,c0861edc,135,ffffffff) at _mtx_lock_flags+0xba
msleep(c20f12e8,c20f1224,5c,c0865441,0) at msleep+0x794
stopevent(c20f11b8,2,13,823,c0922200) at stopevent+0x85
issignal(c1f31bd0,2,c08619f7,bd,1) at issignal+0x168
cursig(c1f31bd0,0,c0864399,104,0) at cursig+0xe8
ast(c9520d48) at ast+0x4b0
doreti_ast() at doreti_ast+0x17
load: 0.21  cmd: strace 583 [iowait] 0.00u 0.91s 0% 724k
[sent a serial break]

Cool, eh?  Second try:

crash2# strace ls
execve(0xbfbfe890, [0xbfbfed54], [/* 0 vars */]PIOCWSTOP: Input/output
error

Even better.

The first obvious observation is that holding mutexes other than the
process mutex over calls to _STOPEVENT() is a bad idea.  It seems like the
p_sig mutex is used to cover a fair amount of flag handling, signal entry
changes, etc, etc.  I'm not familiar with the semantic requirements here,
but presumably something needs to change.  Is it possible to release the
locks after grabbing the value of 'action' (or even do a lock-free read),
and then grab the sigact lock only later during actual delivery, yet
maintain the right semantics?

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert_at_fledge.watson.org      Senior Research Scientist, McAfee Research