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From:   John Ogness <john.ogness@linutronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Petr Mladek <pmladek@suse.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Brendan Higgins <brendanhiggins@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>, Paul Turner <pjt@google.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Prarit Bhargava <prarit@redhat.com>
Subject: Re: printk meeting at LPC
References: <20190905143118.GP2349@hirez.programming.kicks-ass.net>
        <alpine.DEB.2.21.1909051736410.1902@nanos.tec.linutronix.de>
        <20190905121101.60c78422@oasis.local.home>
        <alpine.DEB.2.21.1909091507540.1791@nanos.tec.linutronix.de>
        <87k1acz5rx.fsf@linutronix.de> <20190918012546.GA12090@jagdpanzerIV>
        <20190917220849.17a1226a@oasis.local.home>
        <20190918023654.GB15380@jagdpanzerIV>
        <20190918051933.GA220683@jagdpanzerIV> <87h85anj85.fsf@linutronix.de>
        <20190918081012.GB37041@jagdpanzerIV>
Date:   Wed, 18 Sep 2019 11:05:28 +0200
In-Reply-To: <20190918081012.GB37041@jagdpanzerIV> (Sergey Senozhatsky's
        message of "Wed, 18 Sep 2019 17:10:12 +0900")
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On 2019-09-18, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:
>> Each console has its own iterator. This iterators will need to
>> advance, regardless if the message was printed via write() or
>> write_atomic().
>
> Great.
>
> ->atomic_write() path will make sure that kthread is parked or will
> those compete for uart port?

A cpu-lock (probably per-console) will be used to synchronize the
two. Unlike my RFCv1, we want to keep the cpu-lock out of the console
drivers and we want it to be less aggressive (using trylock's instead of
spinning). This should make the cpu-lock less "dangerous". I talked with
PeterZ, Thomas, and PetrM about how this can be implemented, but there
may still be some corner cases.

I would like to put everything together now so that we can run and test
if the decisions made in that meeting hold up for all the cases. I think
it will be easier to identify/add the missing pieces, once we have it
coded.

John Ogness