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

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Date:   Mon, 9 Sep 2019 00:18:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 0/9] printk: new ringbuffer implementation

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.

So I've been a huge flaming idiot.. so while I'm not particularly
sympathetic to NMIs that block, there are a number of really trivial
deadlocks possible -- and it is a minor miracle I've not actually hit
them (I suppose because printk() isn't really all that common).

The whole cpu-lock thing I had needs to go. But not having it makes
lockless console output unreadable and unsable garbage.

I've got some ideas on a replacement, but I need to further consider it.

:-/