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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 10/25] printk: redirect emit/store to new ringbuffer
References: <20190212143003.48446-1-john.ogness@linutronix.de>
        <20190212143003.48446-11-john.ogness@linutronix.de>
        <20190220090112.xbnauwt2w7gwtebo@pathway.suse.cz>
        <8736oijgpf.fsf@linutronix.de>
        <20190222144302.44zl37p75qgaixf3@pathway.suse.cz>
Date:   Fri, 22 Feb 2019 16:06:26 +0100
In-Reply-To: <20190222144302.44zl37p75qgaixf3@pathway.suse.cz> (Petr Mladek's
        message of "Fri, 22 Feb 2019 15:43:02 +0100")
Message-ID: <87va1byia5.fsf@linutronix.de>
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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