linux-kernel - Re: [RFC PATCH v1 20/25] serial: 8250: implement write

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Date:   Fri, 08 Mar 2019 05:17:36 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 20/25] serial: 8250: implement write_atomic

On 2019-03-08, John Ogness <john.ogness@...utronix.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