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

Steven Rostedt
2018-10-03T01:23:27Z

On Tue, 2 Oct 2018 17:15:17 -0700
Daniel Wang <wonderfly@google.com> wrote:

> On Tue, Oct 2, 2018 at 1:42 AM Petr Mladek <pmladek@suse.com> wrote:
> >
> > Well, I still wonder why it helped and why you do not see it with 4.4.
> > I have a feeling that the console owner switch helped only by chance.
> > In fact, you might be affected by a race in
> > printk_safe_flush_on_panic() that was fixed by the commit:
> >
> > 554755be08fba31c7 printk: drop in_nmi check from printk_safe_flush_on_panic()
> >
> > The above one commit might be enough. Well, there was one more
> > NMI-related race that was fixed by:
> >
> > ba552399954dde1b printk: Split the code for storing a message into the log buffer
> > a338f84dc196f44b printk: Create helper function to queue deferred console handling
> > 03fc7f9c99c1e7ae printk/nmi: Prevent deadlock when accessing the main log buffer in NMI
>
> All of these commits already exist in 4.14 stable, since 4.14.68. The deadlock
> still exists even when built from 4.14.73 (latest tag) though. And cherrypicking
> dbdda842fe96 fixes it.
>

I don't see the big deal of backporting this. The biggest complaints
about backports are from fixes that were added to late -rc releases
where the fixes didn't get much testing. This commit was added in 4.16,
and hasn't had any issues due to the design. Although a fix has been
added:

c14376de3a1 ("printk: Wake klogd when passing console_lock owner")

Also from 4.16, but nothing else according to searching for "Fixes"
tags.

-- Steve