linux-kernel - Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

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Message-ID: <20200923151129.GC6442@alley>
Date:   Wed, 23 Sep 2020 17:11:29 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

On Tue 2020-09-22 17:44:14, John Ogness wrote:
> vprintk_store() is using a single static buffer as a temporary
> sprint buffer for the message text. This will not work once
> @logbuf_lock is removed. Replace the single static buffer with a
> pool of buffers.

The buffer is used because we do not know the length of the
formatted message to reserve the right space in the ring buffer
in advance.

There was the idea to call vsprintf(NULL, fmt, args) to count
the length in advance.

AFAIK, there is one catch. We need to use va_copy() around
the 1st call because va_format can be proceed only once.
See, va_format() in lib/vsprintf.c as an example.

Is there any other problem, please?

Best Regards,
Petr