Re: BUG: corrupted list in __dentry_kill (2)

From: Dmitry Vyukov
Date: Fri Dec 13 2019 - 04:10:41 EST

On Thu, Dec 12, 2019 at 7:34 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Dec 12, 2019 at 04:57:14PM +0100, Dmitry Vyukov wrote:
> > > Speaking of bisect hazards, I'd recommend to check how your bisect
> > > went - the bug is definitely local to this commit and I really
> > > wonder what had caused the bisect to go wrong in this particular
> > > case.
> >
> > I did not get the relation of folding to bisection. Or you mean these
> > are just separate things?
> Suppose instead of folding the fix in I would've done a followup commit
> just with the fix. And left the branch in that form, eventually getting
> it pulled into mainline. From that point on, *ANY* bisect stepping into
> the first commit would've been thrown off. For ever and ever, since
> once it's in mainline, it really won't go away.
> That's what folding avoids - accumulation of scar tissue, if you will.
> Sure, there's enough cases when bug is found too late - it's already
> in mainline or pulled into net-next or some other branch with similar
> "no rebase, no reorder" policy. But if you look at the patchsets posted
> on the lists and watch them from iteration to iteration, you'll see
> a _lot_ of fix-folding. IME (both by my own practice and by watching
> the patchsets posted by others) it outnumbers the cases when fix can't
> be folded by quite a factor. I wouldn't be surprised if it was an
> order of magnitude...
> Strict "never fold fixes" policy would've accelerated the accumulation
> of bisect hazards in the mainline. And while useful bisect may be a lost
> cause for CI bots, it isn't that for intelligent developers. Anything
> that makes it more painful is not going to be welcome.

Ah, I see. Yes, folding will help future bisections. In fact, an
unfolded bug somewhere in kernel history is exactly what caused wrong
result on bisection for this bug.
Just in case, I did not propose to not do folding here (or anywhere
else as far as I remember). Handling of folded fixes is documented in
syzbot docs: