Date: Thu, 11 Sep 2008 10:28:59 -0400 From: John Baldwin <jhb@freebsd.org> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: arch@freebsd.org Subject: Re: PASSERT() - asserting for panics Message-ID: <200809111028.59610.jhb@freebsd.org> In-Reply-To: <200809110006.m8B06vOU033199@apollo.backplane.com> References: <200809101531.54646.jhb@FreeBSD.org> <200809101806.38042.jhb@freebsd.org> <200809110006.m8B06vOU033199@apollo.backplane.com>
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On Wednesday 10 September 2008 08:06:57 pm Matthew Dillon wrote:
>
> :I think part of the pollution is that what I really want to do is treat a
> :panic like an exception that I can catch. I could probably make that idiom
> :work with two macros so your code can be:
> :
> : PANIC_TRY {
> : /* do stuff */
> : } PANIC_CATCH("foo");
> :
> :That could potentially work with some evil goto's that jumped between the
> :macros (i.e. have the setjmp() in PANIC_CATCH and PANIC_TRY goto's down to
> :--
> :John Baldwin
>
> Hmm. Almost java-like, where an exception pops back through subroutine
> levels until it finds a match. That kind of functionality would be a
> real mess in C. A limited form would be possible, something like this:
>
> #define PANIC_CATCH(label) \ (...)
> static jmpbuf label # _jmpbuf
> if (0) {
> for (;;) {
> longjmp(&label # _jmpbuf);
> label:
>
> #define PANIC_RETRY }}
>
> #define PANIC_PANIC panic(...); }}
>
> #define PASSERT(label, cond) \ (...)
> if (__predict_false(!(cond))) {
> setjmp (&label # _jmpbuf);
> goto label;
> }
>
> PANIC_CATCH(badthings) {
> ...
> PANIC_RETRY;
> }
>
> PANIC_CATCH(badthings) {
> ...
> PANIC_PANIC;
> }
>
> PASSERT(badthings, x == 1);
The problem is I want panics in unmodified code to be caught. For example, I
want to write regression tests to make sure assertions on locking primivities
fail when the state of the lock doesn't match what the assertion is
requiring. I can do something fairly simple though that lets me do this:
PANIC_TRY {
stuff;
} PANIC_CATCH {
IGNORE_PANIC("this one is ok");
/* WITNESS can do different panics in foo_assert() sometimes */
IGNORE_PANIC("this one is too");
UNEXPECTED_PANIC();
}
Without needing goto's.
> Which would allow you some control over the context plus allow multiple
> PASSERT's with the same label. But, OMG I don't know about doing
> setjmp/longjmp in the kernel. I don't think it would be worth it.
We use setjmp already if you get a trap in ddb so ddb doesn't crash.
> Theoretically one could cross procedural boundaries with the longjmp,
> and place the jmpbuf in a DATASET and have the kernel glue the longjmp
> address. That would be a candidate for the C obfuscation contest
> though.
To catch unmodified panics, I have to have the longjmp in panic() itself, and
I have the jmpbuf hung off of curthread.
--
John Baldwin
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