Re: [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk header

Petr Mladek
2018-10-16T11:40:06Z

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@gmail.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