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

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Message-ID: <20200924085343.GB541@jagdpanzerIV.localdomain>
Date:   Thu, 24 Sep 2020 17:53:43 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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 (20/09/24 10:45), Petr Mladek wrote:
> On Thu 2020-09-24 14:40:58, Sergey Senozhatsky wrote:
> > On (20/09/23 17:11), Petr Mladek wrote:
> > >
> > > AFAIK, there is one catch. We need to use va_copy() around
> > > the 1st call because va_format can be proceed only once.
> > >
> > 
> > Current printk() should be good enough for reporting, say, "Kernel
> > stack overflow" errors. Is extra pressure that va_copy() adds something
> > that we need to consider?
> 
> The thing is that vsprintf() traverses the arguments using va_arg().
> It modifies internal values so that the next va_arg() will read
> the next value.

Yes, I understand the purpose of va_copy(). I'm asking if we are
always on the safe side doing va_copy for every printk (+ potential
recursive va_copy-s).

> If we want to call vsprintf() twice then we need to reset the internal
> va_list states. My understanding is that va_copy() is the only legal
> way when we are already nested inside va_start()/va_end().

Yes, it is. My question is a bit different.

	-ss