Re: 4.14 backport request for dbdda842fe96f: "printk: Add console owner and waiter logic to load balance console writes"

Pavel Machek
2018-10-02T02:52:41Z

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On Mon 2018-10-01 15:23:24, Steven Rostedt wrote:
> On Thu, 27 Sep 2018 12:46:01 -0700
> Daniel Wang <wonderfly@google.com> wrote:
>
> > Prior to this change, the combination of `softlockup_panic=1` and
> > `softlockup_all_cpu_stacktrace=1` may result in a deadlock when the reboot path
> > is trying to grab the console lock that is held by the stack trace printing
> > path. What seems to be happening is that while there are multiple CPUs, only one
> > of them is tasked to print the back trace of all CPUs. On a machine with many
> > CPUs and a slow serial console (on Google Compute Engine for example), the stack
> > trace printing routine hits a timeout and the reboot path kicks in. The latter
> > then tries to print something else, but can't get the lock because it's still
> > held by earlier printing path. This is easily reproducible on a VM with 16+
> > vCPUs on Google Compute Engine - which is a very common scenario.
> >
> > A quick repro is available at
> > https://github.com/wonderfly/printk-deadlock-repro. The system hangs 3 seconds
> > into executing repro.sh. Both deadlock analysis and repro are credits to Peter
> > Feiner.
> >
> > Note that I have read previous discussions on backporting this to stable [1].
> > The argument for objecting the backport was that this is a non-trivial fix and
> > is supported to prevent hypothetical soft lockups. What we are hitting is a real
> > deadlock, in production, however. Hence this request.
> >
> > [1] https://lore.kernel.org/lkml/20180409081535.dq7p5bfnpvd3xk3t@pathway.suse.cz/T/#u
> >
> > Serial console logs leading up to the deadlock. As can be seen the stack trace
> > was incomplete because the printing path hit a timeout.
>
> I'm fine with having this backported.

Dunno. Is the patch perhaps a bit too complex? This is not exactly
trivial bugfix.

pavel@duo:/data/l/clean-cg$ git show dbdda842fe96f | diffstat
printk.c | 108
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-

I see that it is pretty critical to Daniel, but maybe kernel with
console locking redone should no longer be called 4.4?
Pavel
--
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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