diary at Telent Netowrks


Tue, 23 Jul 2002 17:29:52 +0000


The FreeBSD problem was that it really didn't like us fixing up the stack pointer (to swap back to the normal control stack) by hand in assembly: when it's about to call the signal handler on subsequent occasions, it seems to remember that it's still on the alternate stack - note if you will that we aren't actually ever returning from the signal handler, we're just calling into lisp and letting lisp unwind past it. I assume it's storing the running-on-external-stack-p property somewhere instead of just comparing the stack pointer with the bounds of the alternate stack.

Either we fix this somehow, probably involving the perusal of FreeBSD kernel sources, or we think of a new approach. The latter sounds tempting ...

Here's how to do it, then.

  1. In the signal handler, fake a control stack frame in much the same way as fakeforeignfunction_call - the function that prepares us for calling into Lisp after a signal is received - would have done. This is actually a no-op on x86 anyway.

  2. Frob the PC in the signal context, plus sundry other registers to make it look plausible, so that the signal handler "returns" to our error-raising function. This is the lovely piece of code

           &(((struct simple_fun *)
        *os_context_pc_addr(context)= function;

  3. Amazingly, this works. And it lets us delete all that awful x86 assembler, which pleases me no end as I feared that I'd be stuck answering questions when it turned out to be buggy.

Patch going to sbcl-devel mailing list and/or being committed (I'll decide which after I've read it again and formed an opinion on the essential/accidental grottiness ratio)