Made a certain amount of what I think is progress on the GC issue.
The pseudo-atomic mechanism in SBCL is supposed to let you run short sequences of Lisp code without icky things happening if signals are received during them. What it does is set a flag at the start of the PA sequence; any signal handler is supposed to check this and say "ah, unsafe. Let's save the information that this signal went off, and the signal mask as it was before the signal (from the signal context), then block all signals and return immediately". At the end of the PA sequence we check whether a signal had been received. If so, we run a trap instruction which sends us SIGTRAP which runs sigtrap_handler, which eventually gets around to running the signal handler for the pending signal. We also hack up the sigmask in the signal context to be whatever we'd saved when the pseudo-atomic section was interrupted, so when this handler returns, the process will once again be running with the normal mask, which is probably "nothing blocked". Clear? Good. I have to think about it quite hard too.
The net result should be pretty much as it would be if we just masked signals for the duration, but in the normal case that no signal was sent, we don't have the expense of a couple of syscalls to disable/reenable. Note that we don't have room to save more than one signal this way, but that's OK, because we block them all as soon as the first one goes off.
One of the new signals we've added recently (in fact, I haven't even committed this yet, that's how recent) is SIGSTOPFORGC (actually SIGRTMIN+n), which is half of the protocol for pausing threads when we need to vacuum underneath them. The thread that wants to GC sends SIGSTOPFORGC to all the others. They have a sigstopforgchandler which does the pseudo-atomic thing, then kill(thisthread->pid,SIGSTOP). The first point of note is that this signal is not in the set that gets blocked during pseudo-atomic. So, if a thread in PA gets some other signal followed by a request to stop for GC, the STOPFOR_GC signal might overwrite the pending signal data. So, add it to the list.
But this means we can't do any kind of likely-to-block operation with signals masked, because the GC will want to pause us while we do it, and if we ignore its signal it will sit there (looping) until we take note. Ungood. Likely-to-block operations include waiting on queues, so anything involving lock acquisition is out.
Allocation is sometimes done with signals masked. I know that allocations from C inside an error handler or similar will do this, but maybe there's lisp code that also does. Allocation is pseudo-atomic: if we allocate with signals blocked, and the allocator decides to arrange for a garbage collection to happen after we're done, the saved signal mask in the pending information will have signals blocked too, so, well, the upshot is that we will run SUB-GC (the Lisp-level GC routine that calls collect_garbage) with, guess what, blocked signals. This is bad because it contains a with-mutex form to ensure that only one thread is collecting at a time.
This is looking less and less likely to be fixed in time for reasonable testing before 0.8.4.
Anyway, we can now at least unblock before calling SUB-GC, and it makes some kind of (positive, I think) difference. There's still more to do, though. (1) I've managed one time to get a thread looping in get-mutex, (2) it's hit against my new debugging assertion that says "don't sleep on a queue with signals disabled", but in this case the only two blocked signals are SIGTRAP and SIG_DEQUEUE (actually SIGRTMIN+n'), and (3) SIGSEGV is not one of the usual blocking set. I don't know if this is because (a) it can never happen in pseudo-atomic code - though SIGSEGV as a write barrier for objects in old spaces can happen just about anywhere, as far as I can see, or (b) because it didn't occur to the authors that SIGSEGV can occur just about anywhere - the code might well predate the generational gc and not have been updated, or (c) for some other reason I still don't understand. (b) or (c) seem likely.
What would block SIGTRAP and SIG_DEQUEUE? We don't even have a signal handler for the latter: we use sigwait() for it instead.