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

Sergey Senozhatsky
2018-12-12T08:10:34Z

On (12/12/18 01:48), Sasha Levin wrote:
> > > > I guess we still don't have a really clear understanding of what exactly
> > > is going in your system
> > >
> > > I would also like to get to the bottom of it. Unfortunately I haven't
> > > got the expertise in this area nor the time to do it yet. Hence the
> > > intent to take a step back and backport Steven's patch to fix the
> > > issue that has resurfaced in our production recently.
> >
> > No problem.
> > I just meant that -stable people can be a bit "unconvinced".
>
> The -stable people tried adding this patch back in April, but ended up
> getting complaints up the wazoo (https://lkml.org/lkml/2018/4/9/154)
> about how this is not -stable material.

OK, really didn't know that! I wasn't Cc-ed on that AUTOSEL email,
and I wasn't Cc-ed on this whole discussion and found it purely
accidentally while browsing linux-mm list.

I understand what Petr meant by his email. Not arguing; below are just
my 5 cents.

> So yes, testing/acks welcome :)

OK. The way I see it (and I can be utterly wrong here):

The patch set in question, most likely and probably (*and those are
theories*), makes panic() deadlock less likely because panic_cpu waits
for console_sem owner to release uart_port/console_owner locks before
panic_cpu pr_emerg("Kernel panic - not syncing"), dump_stack()-s and
brings other CPUs down via stop IPI or NMI.
So a precondition is
panic CPU != uart_port->lock owner CPU

If the panic happens on the same CPU which holds the uart_port spin_lock,
then the deadlock is still there, just like before; we have another patch
which attempts to fix this (it makes console drivers re-entrant from
panic()).

So if you are willing to backport this set to -stable, then I wouldn't
mind, probably would be more correct if we don't advertise this as a
"panic() deadlock fix" tho; we know that deadlock is still possible.
And there will be another -stable backport request in a week or so.

In the meantime, I can add my Acked-by to this backport if it helps.

/* Assuming that my theories explain what's happening with
Daniel's systems. */

-ss