linux-kernel - Re: [RFC][PATCHv2 2/4] printk: move printk

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Date:   Tue, 16 Oct 2018 13:40:06 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        linux-serial@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk
 header

On Tue 2018-10-16 09:27:19, Peter Zijlstra wrote:
> On Tue, Oct 16, 2018 at 02:04:26PM +0900, Sergey Senozhatsky wrote:
> > Make printk_safe_enter_irqsave()/etc macros available to the
> > rest of the kernel.
> > 
> > Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > ---
> >  include/linux/printk.h      | 40 +++++++++++++++++++++++++++++++++++++
> >  kernel/printk/internal.h    | 37 ----------------------------------
> >  kernel/printk/printk_safe.c |  6 ++++--
> >  3 files changed, 44 insertions(+), 39 deletions(-)
> > 
> > diff --git a/include/linux/printk.h b/include/linux/printk.h
> > index cf3eccfe1543..75f99441fd54 100644
> > --- a/include/linux/printk.h
> > +++ b/include/linux/printk.h
> > @@ -157,6 +157,46 @@ static inline void printk_nmi_direct_enter(void) { }
> >  static inline void printk_nmi_direct_exit(void) { }
> >  #endif /* PRINTK_NMI */
> >  
> > +#ifdef CONFIG_PRINTK
> > +extern void printk_safe_enter(void);
> > +extern void printk_safe_exit(void);
> > +
> > +#define printk_safe_enter_irqsave(flags)	\
> > +	do {					\
> > +		local_irq_save(flags);		\
> > +		printk_safe_enter();		\
> > +	} while (0)
> > +
> > +#define printk_safe_exit_irqrestore(flags)	\
> > +	do {					\
> > +		printk_safe_exit();		\
> > +		local_irq_restore(flags);	\
> > +	} while (0)
> > +
> > +#define printk_safe_enter_irq()		\
> > +	do {					\
> > +		local_irq_disable();		\
> > +		printk_safe_enter();		\
> > +	} while (0)
> > +
> > +#define printk_safe_exit_irq()			\
> > +	do {					\
> > +		printk_safe_exit();		\
> > +		local_irq_enable();		\
> > +	} while (0)
> 
> So I really _really_ hate all this. Utterly detest it.
> 
> That whole magic 'safe' printk buffer crap is just that crap.
> 
> Instead of this tinkering around the edges, why don't you make the main
> logbuf a lockless ringbuffer and then delegate the actual printing of
> that buffer to a kthread, except for earlycon, which can do synchronous
> output.

In fact, there is no problem with printk log buffer. This patchset
tries to avoid deadlock caused by console-specific locks
used by console->write() methods.

By other words, we neither need printk_safe or lockless log buffer
to fix this prolem. Instead, we need either printk_deferred context
or lockless consoles.

Best Regards,
Petr