From mboxrd@z Thu Jan  1 00:00:00 1970
From: John Ogness <john.ogness@linutronix.de>
Subject: Re: [RFC PATCH v1 10/25] printk: redirect emit/store to new ringbuffer
Date: Fri, 22 Feb 2019 16:06:26 +0100
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In-Reply-To: <20190222144302.44zl37p75qgaixf3@pathway.suse.cz> (Petr Mladek's
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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>
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On 2019-02-22, Petr Mladek <pmladek@suse.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