Re: [RFC PATCH v4 0/9] printk: new ringbuffer implementation

From: Peter Zijlstra
Date: Fri Sep 06 2019 - 10:22:52 EST

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.