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From:   John Ogness <john.ogness@linutronix.de>
To:     Petr Mladek <pmladek@suse.com>
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Wang <wonderfly@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@suse.com>,
        Peter Feiner <pfeiner@google.com>,
        linux-serial@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v1 20/25] serial: 8250: implement write_atomic
References: <20190212143003.48446-1-john.ogness@linutronix.de>
        <20190212143003.48446-21-john.ogness@linutronix.de>
        <20190227094655.ecdwhsc2bf5spkqx@pathway.suse.cz>
        <87zhqhed3u.fsf@linutronix.de>
        <20190227135504.gqtcsdwpy4rd7xvs@pathway.suse.cz>
        <87a7i6ovt3.fsf@linutronix.de>
Date:   Fri, 08 Mar 2019 05:17:36 +0100
In-Reply-To: <87a7i6ovt3.fsf@linutronix.de> (John Ogness's message of "Fri, 08
        Mar 2019 05:05:12 +0100")
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On 2019-03-08, John Ogness <john.ogness@linutronix.de> wrote:
> If the ringbuffer was fully lockless, we should be able to have
> per-console CPU-reentrant spin locks as long as the ordering is
> preserved, which I expect shouldn't be a problem. If any NMI context
> needed a spin lock for its own purposes, it would need to use the
> CPU-reentrant spin lock of the first console so as to preserve the
> ordering in case of a panic.

This point is garbage. Sorry. I do not see how we could safely have
multiple CPU-reentrant spin locks. Example of a deadlock:

CPU0                  CPU1
printk                printk
  console2.lock         console1.lock
NMI                   NMI
  printk                printk
    console1.lock         console2.lock

>> ... it should not be a common lock for the ring buffer and all
>> consoles.
>
> If the ring buffer becomes fully lockless, then we could move to
> per-console CPU-reentrant spin locks.

A fully lockless ring buffer will reduce the scope of the one, global
CPU-reentrant spin lock. But I do not see how we can safely have
multiple of these. If it is part of printk, it is already implicitly on
every line of code.

John Ogness