linux-kernel - Re: current git kernel has strange problems during bisect

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Date:	Sun, 11 Jan 2009 17:34:51 -0500 (EST)
From:	Daniel Barkalow <barkalow@...ervon.org>
To:	Alexey Zaytsev <alexey.zaytsev@...il.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Johannes Schindelin <Johannes.Schindelin@....de>,
	git@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: current git kernel has strange problems during bisect

On Mon, 12 Jan 2009, Alexey Zaytsev wrote:

> On Sun, Jan 11, 2009 at 23:04, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> >
> > On Sun, 11 Jan 2009, Sam Ravnborg wrote:
> >>
> >> The cost of moving this piece of history from one git tree to another
> >> git tree is that we make it harder to debug the kernel for the advanced user
> >> that knows how to do bisect.
> >>
> >> It is not like this history would be lost - one just had to look
> >> somewhere else to find it.
> >>
> >> That may be a bad pain/benefit ratio - time will tell.
> >
> > Umm. No.
> >
> > Time is exactly what makes it useful. It will make all the downsides
> > shrink, and the advantages stay.
> >
> >> There should be a way to avoid such pain when bisecting without
> >> having to mark a semi-random (for the average person) commit as good.
> >
> > Well, you don't actually have to mark that semi-random one as good either.
> > What you can do is to just mark anything that _only_ contains fs/btrfs as
> > good. IOW, you don't have to know the magic number - you just have to be
> > told that "oh, if you only have btrfs files, and you're not actively
> > bisecting a btrfs bug, just do 'git bisect good' and continue".
> >
> > Yeah, you'll hit it a few times, but you don't even have to compile things
> > or boot anything, so it's not actually going to be all that much slower
> > than just knowing about the magic point either.
> 
> But would not such bug avoid being bisected if you blindly
> mark btrfs commits as good?
> 
> v2.6.29 <-- bad
> ...
> ...
> ...
> btrfs stuff <-- mark as good
> ...
> the-real-bug
> ...
> v2.6.28 <-- good
> 
> So you hit the btrfs commit, mark it as good, leaving the real bug below,
> and the bisection continues, with both sides being actually bad.
> 
> Am I missing something?

Yes, there are no kernel bugs below the btrfs stuff, because there's no 
kernel at all below the btrfs stuff. The history is actually like:

A -- B -- C -- D -- G
              /
        F -- E

F and E are the btrfs stuff, while A-D and G are commit containing the 
kernel source (D and G also containing btrfs). Marking E as good cuts off 
F, but doesn't cut off anything at all on the top line. Of course, if 
you're actually debugging a problem with btrfs that you somehow know to 
have worked while btrfs was a separate module at so point, you would want 
to get into this history (and would build it as a separate module in order 
to do so).

	-Daniel
*This .sig left intentionally blank*
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