linux-kernel - Re: Can printk() sleep at runtime?

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Date:   Thu, 31 May 2018 17:13:08 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Stephen Boyd <sboyd@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>, baijiaju1990@...il.com,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        SergeySenozhatsky <sergey.senozhatsky@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Michael Turquette <mturquette@...libre.com>
Subject: Re: Can printk() sleep at runtime?

On Thu, 31 May 2018 18:42:48 +0200
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Hi Stephen,
> 
> On Thu, May 31, 2018 at 5:19 PM, Stephen Boyd <sboyd@...nel.org> wrote:
> > Quoting Linus Torvalds (2018-05-31 07:32:10)  
> >> On Thu, May 31, 2018 at 5:05 AM Petr Mladek <pmladek@...e.com> wrote:  
> >> > Anyway, we need to fix or remove this format. vsprintf-like functions
> >> > are called in any context and nobody expect that they might sleep.  
> >>
> >> Ack. I guess the argument is that "%pCr" is rare, and none of *those*
> >> users may care, but I do think that doing things wrong as-is.
> >>
> >> It's too subtle to have to know you're in a particular locking context
> >> when you use a particular %p modifier.  
> >
> > Agreed. Removing the format seems to be the best approach. It looks like
> > only Geert has used it in the last few years and it hasn't been used
> > much otherwise.  
> 
> Indeed, just 3 users (the broadcom one isn't mine):
>     drivers/clk/renesas/renesas-cpg-mssr.c
>     drivers/thermal/broadcom/bcm2835_thermal.c
>     drivers/tty/serial/sh-sci.c
> 
> Alternatively, can we have a special version __clk_get_rate() that just
> returns clk->core->rate?
> Or would that be too inaccurate in the presence of CLK_GET_RATE_NOCACHE?
> The function could still return 0 in case the flag is set.

If it's only used in three locations, I think it would be better to
simply remove it from vsprintf() and have the three callers call
clk_get_rate() directly.

-- Steve