Re: WARNING in up_write

[Dave Chinner]

On Thu, Apr 05, 2018 at 05:13:25PM -0700, Eric Biggers wrote:
> On Fri, Apr 06, 2018 at 08:32:26AM +1000, Dave Chinner wrote:
> > On Wed, Apr 04, 2018 at 08:24:54PM -0700, Matthew Wilcox wrote:
> > > On Wed, Apr 04, 2018 at 11:22:00PM -0400, Theodore Y. Ts'o wrote:
> > > > On Wed, Apr 04, 2018 at 12:35:04PM -0700, Matthew Wilcox wrote:
> > > > > On Wed, Apr 04, 2018 at 09:24:05PM +0200, Dmitry Vyukov wrote:
> > > > > > On Tue, Apr 3, 2018 at 4:01 AM, syzbot
> > > > > > <syzbot+dc5ab2babdf22ca091af@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > > DEBUG_LOCKS_WARN_ON(sem->owner != get_current())
> > > > > > > WARNING: CPU: 1 PID: 4441 at kernel/locking/rwsem.c:133 up_write+0x1cc/0x210
> > > > > > > kernel/locking/rwsem.c:133
> > > > > > > Kernel panic - not syncing: panic_on_warn set ...
> > > > > 
> > > > > Message-Id: <1522852646-2196-1-git-send-email-longman@xxxxxxxxxx>
> > > > >
> > > > 
> > > > We were way ahead of syzbot in this case.  :-)
> > > 
> > > Not really ... syzbot caught it Monday evening ;-)
> > 
> > Rather than arguing over who reported it first, I think that time
> > would be better spent reflecting on why the syzbot report was
> > completely ignored until *after* Ted diagnosed the issue
> > independently and Waiman had already fixed it....
> > 
> > Clearly there is scope for improvement here.
> > 
> > Cheers,
> > 
> 
> Well, ultimately a human needed to investigate the syzbot bug report to figure
> out what was really going on.  In my view, the largest problem is that there are
> simply too many bugs, so many are getting ignored.

Well, yeah. And when there's too many bugs, looking at the ones
people are actually hitting tend to take precedence over those
reported by a bot an image problem...

> If there were only a few bugs, then Dmitry would investigate each
> one and send a "real" bug report of better quality than the
> automated system can provide, or even send a fix directly.  But in
> reality, on the same day this bug was reported, syzbot also found
> 10 other bugs, and in the previous 2 days it had found 38 more.
> No single person can keep up with that. 

And this is precisely why people turn around and ask the syzbot
developers to do things that make it easier for them to diagnose
the problems syzbot reports.

> You can see the current
> bug list, which has 172 open bugs, on the dashboard at
> https://syzkaller.appspot.com/. 

Is that all? That's *nothing*.

> Yes, the kernel really is that
> broken.

Actually, that tells me the kernel is a hell of a lot better than my
experience leads me to beleive it is. I'd have expected thousands of
bugs, even tens of thousands of bugs given how many issues we deal
with in individual subsystems on a day to day basis.

> And although quite a few of these bugs will end up to be
> duplicates or even already fixed, a human still has to look at
> each one to figure that out.  (Though, I do think that syzbot
> should try to automatically detect when a reproducible bug was
> already fixed, via bisection.  It would cause a few bugs to be
> incorrectly considered fixed, but it may be a worthwhile
> tradeoff.)
> 
> These bugs are all over the kernel as well, so most developers
> don't see the big picture but rather just see a few bugs for
> "their" subsystem on "their" subsystem's mailing list and
> sometimes demand special attention.  Of course, it's great when
> people suggest ways to improve the process.

That's not the response I got....

> But it's not great
> when people just don't feel responsible for fixing bugs and wait
> for Someone Else to do it.

The excessive cross posting of the reports is one of the reasons
people think someone else will take care of it. i.e. "Oh, that looks VFS,
that went to -fsdevel, I don't need to look at it"....

Put simply: if you're mounting an XFS filesystem image and something
goes bang, then it should be reported to the XFS list. It does not
need to be cross posted to LKML, -fsdevel, 10 individual developers,
etc. If it's not an XFS problem, then the XFS developers will CC the
relevant lists as needed.

> I'm hoping that in the future the syzbot "team", which seems to
> actually be just Dmitry now, can get more resources towards
> helping fix the bugs.  But either way, in the end Linux is a
> community effort.

We don't really need help fixing the bugs - we need help making it
easier to *find the bug* the bot tripped over. That's what the
syzbot team needs to focus on, not tell people that what they got is
all they are going to get.

> Note also that syzbot wasn't super useful in this particular case
> because people running xfstests came across the same bug.  But,
> this is actually a rare case.  Most syzbot bug reports have been
> for weird corner cases or races that no one ever thought of
> before, so there are no existing tests that find them.

Which is exactly what these whacky "mount a filesystem fragment"
tests it is now doing are exercising.  Finding the cause of
corruption related crashes is not easy and takes time. Having the
bot developers add something to the bot that will save the developer
looking at the problem 10 minutes of setup time makes a huge
difference to the effort required to find the problem.

The tool is useless if people find it too hard to make sense of the
bug reports (*cough* lockdep *cough*) or perform triage of the
report. If we want to get the bugs fixed faster, we have to make the
reports from automated tools contain the exact information the
developer needs to solve the problem.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx