linux-kernel - Re: [RFC PATCH v1 10/25] printk: redirect emit/store to new ringbuffer

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Date:   Fri, 22 Feb 2019 16:06:26 +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 10/25] printk: redirect emit/store to new ringbuffer

On 2019-02-22, Petr Mladek <pmladek@...e.com> wrote:
>>>> +	rbuf = prb_reserve(&h, &sprint_rb, PRINTK_SPRINT_MAX);
>>>
>>> The second ring buffer for temporary buffers is really interesting
>>> idea.
>>>
>>> Well, it brings some questions. For example, how many users might
>>> need a reservation in parallel. Or if the nested use might cause
>>> some problems when we decide to use printk-specific ring buffer
>>> implementation. I still have to think about it.
>> 
>> Keep in mind that it is only used by the writers, which have the
>> prb_cpulock. Typically there would only be 2 max users: a non-NMI
>> writer that was interrupted during the reserve/commit window and the
>> interrupting NMI that does printk. The only exception would be if the
>> printk-code code itself triggers a BUG_ON or WARN_ON within the
>> reserve/commit window. Then you will have an additional user per
>> recursion level.
>
> I am not sure it is worth to call the ring buffer machinery just
> to handle 2-3 buffers.

It may be slightly overkill, but:

1. We have the prb_cpulock at this point anyway, so it will be
   fast. (Both ring buffers share the same prb_cpulock.)

2. Getting a safe buffer is just 1 line of code: prb_reserve()

3. Why should we waste _any_ lines of code implementing the handling of
   these special 3-4 buffers?

> Well, it might be just my mental block. We need to be really careful
> to avoid infinite recursion when storing messages into the log
> buffer.

The recursion works well. I inserted a triggerable BUG_ON() in
vprintk_emit() _within_ the reserve/commit window and I see a clean
backtrace on the emergency console.

John Ogness