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SubjectRe: [patch 00/29] lockup_detector: Cure hotplug deadlocks and replace duct tape
On Thu, 31 Aug 2017 18:10:14 -0400
Don Zickus <dzickus@redhat.com> wrote:

> On Thu, Aug 31, 2017 at 09:15:58AM +0200, Thomas Gleixner wrote:
> > The lockup detector is broken is several ways:
> >
> > - It's deadlock prone vs. CPU hotplug in various ways. Some of these
> > are due to recursive cpus_read_lock() others are due to
> > cpus_read_lock() from CPU hotplug callbacks which immediately lock
> > the machine because cpus are write locked.
> >
> > - The handling of the cpu hotplug threads happens sideways to the
> > smpboot thread infrastructure, which is racy and pointless
> >
> > - The handling of the user space sysctl interface is a complete
> > trainwreck as it fiddles directly with variables which can be
> > modified or evaluated by the running watchdogs.
> >
> > - The perf event initialization is a steaming pile of duct tape as it
> > idiotically tries to create perf events over and over even if perf is
> > not functional (no hardware, ....). To avoid excessive dmesg spam it
> > contains magic printk ratelimiting along with either wrong or useless
> > messages.
> >
> > - The code structure is horrible as ifdef sections are scattered all
> > over the place which makes it unreadable
> >
> > - There is more wreckage, but see the changelogs for the ugly details.
> >
> > Before I get utterly grumpy, I just pretend that I don't give a sh*t!
> >
> > The following series sanitizes the facility and addresses the problems.
>
> Hi Thomas,
>
> Thanks for the patchset. I agree with most your issues you complained
> about, just wasn't smart enough to figure out the right way to solve them.
> Despite your aggressive comments, I will review the code to see if it covers
> the scenarios that have popped up over the years and run some testing on my
> side. Probably need a few days to do that.

The powerpc bits look fine, there's no real changes pending there,
so just take them through your tree if you like.

I had a glance throught the series, no comments yet. The powerpc watchdog
already duplicates the proc tunables rather than using them directly, so
in theory it did not need the 2 stage reconfigure. In practice, it has a
brown paper bag bug because it does not stop the watchdog before changing
its internal variables :P 2 stage is probably safer and clearer way to go
though.

Thanks,
Nick

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