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

Petr Mladek
2018-10-02T08:42:25Z

On Mon 2018-10-01 13:37:30, Daniel Wang wrote:
> On Mon, Oct 1, 2018 at 12:23 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > > 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.
>
> Thanks. I can send the cherrypicks your way. Do you recommend that I
> include the three follow-up fixes though?
>
> c14376de3a1b printk: Wake klogd when passing console_lock owner
> fd5f7cde1b85 printk: Never set console_may_schedule in console_trylock()
> c162d5b4338d printk: Hide console waiter logic into helpers
> dbdda842fe96 printk: Add console owner and waiter logic to load
> balance console writes

This list looks complete and I am fine with backporting it to 4.14.

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

Best Regards,
Petr