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Date:   Fri, 6 Sep 2019 16:22:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Andrea Parri <andrea.parri@amarulasolutions.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Brendan Higgins <brendanhiggins@google.com>,
        John Ogness <john.ogness@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v4 0/9] printk: new ringbuffer implementation
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On Fri, Sep 06, 2019 at 04:01:26PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 06, 2019 at 02:42:11PM +0200, Petr Mladek wrote:
> > 7. People would complain when continuous lines become less
> >    reliable. It might be most visible when mixing backtraces
> >    from all CPUs. Simple sorting by prefix will not make
> >    it readable. The historic way was to synchronize CPUs
> >    by a spin lock. But then the cpu_lock() could cause
> >    deadlock.
> 
> Why? I'm running with that thing on, I've never seen a deadlock ever
> because of it. In fact, i've gotten output that is plain impossible with
> the current junk.
> 
> The cpu-lock is inside the all-backtrace spinlock, not outside. And as I
> said yesterday, only the lockless console has any wait-loops while
> holding the cpu-lock. It _will_ make progress.

Oooh, I think I see. So one solution would be to pass the NMI along in
chain like.  Send it to a single CPU at a time, when finished, send it
to the next.