Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Sat, Jul 23, 2016 at 02:39:52PM +0900, Linus Torvalds wrote:
> On Sat, Jul 23, 2016 at 2:35 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > While doing the scanning and printing, it does call the frame pointer
> > unwinder in parallel, but like before, that's *only* used to determine
> > whether a found address should be printed without a question mark.  If
> > the unwinder goes off the rails, the scanning and printing of text
> > addresses goes on, undisturbed.
> >
> > The frame pointer unwinder code itself is quite careful not to
> > dereference anything it shouldn't (though of course I welcome any review
> > comments that find otherwise).
> 
> So this was the bug the last time around we did unwinders - the code
> would dereference the unwind tables, and the tables would be
> corrupted. End result: recursive oops.
> 
> And they were corrupted not even because of memory corruption, but
> simply because they contained incorrect data, due to compiler bugs and
> other issues.
> 
> I have really bad memories from that time. Several years after the
> fact. It took months to finally revert the crap, because the author
> continued to insist that "this was the last bug" for several passes
> through that thing.
> 
> As they say, "Once burned, twice shy". But in this case, it's more
> like "Four times burned, sixteen times as shy".

But that was DWARF, right?  This is still just simple frame pointers.

Don't think of it as a new unwinder.  Think of it instead as a "gentle
reshuffling of the existing code to vastly improve readability and
maintenance."

Yes, I would like to eventually propose a DWARF unwinder, which
hopefully learns from the mistakes of previous attempts.  But either
way, I think this patch set stands on its own as a big improvement.

-- 
Josh