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

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Message-ID: <20181002084225.6z2b74qem3mywukx@pathway.suse.cz>
Date:   Tue, 2 Oct 2018 10:42:25 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Daniel Wang <wonderfly@...gle.com>
Cc:     rostedt@...dmis.org, stable@...r.kernel.org,
        Alexander.Levin@...rosoft.com, akpm@...ux-foundation.org,
        byungchul.park@....com, dave.hansen@...el.com, hannes@...xchg.org,
        jack@...e.cz, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mathieu.desnoyers@...icios.com, mgorman@...e.de, mhocko@...nel.org,
        pavel@....cz, penguin-kernel@...ove.sakura.ne.jp,
        peterz@...radead.org, tj@...nel.org, torvalds@...ux-foundation.org,
        vbabka@...e.cz, xiyou.wangcong@...il.com,
        Peter Feiner <pfeiner@...gle.com>
Subject: Re: 4.14 backport request for dbdda842fe96f: "printk: Add console
 owner and waiter logic to load balance console writes"

On Mon 2018-10-01 13:37:30, Daniel Wang wrote:
> On Mon, Oct 1, 2018 at 12:23 PM Steven Rostedt <rostedt@...dmis.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