Re: [RFC][PATCHv5 3/7] printk: introduce per-cpu safe_print seq buffer

Sergey Senozhatsky
2016-12-12T14:12:42Z

On (12/12/16 14:54), Petr Mladek wrote:
> On Sat 2016-12-10 12:10:22, Sergey Senozhatsky wrote:
> > On (12/09/16 17:46), Petr Mladek wrote:
> > > > -/*
> > > > - * Safe printk() for NMI context. It uses a per-CPU buffer to
> > > > - * store the message. NMIs are not nested, so there is always only
> > > > - * one writer running. But the buffer might get flushed from another
> > > > - * CPU, so we need to be careful.
> > > > - */
> > >
> > > We should keep/create a good description here because the function
> > > has a non-trivial code. What about something like?
> > >
> >
> > which is really not related to this patch set.
>
> I am sorry but I do not understand. This patch removes description
> that explained constrains of a rather complex code. In fact, the
> constrains has changed because we started using the function also
> in other context. When will be the right time/patchset to explain
> it?

but I didn't remove it.

$ grep -A3 -B3 'But the buffer might get flushed from another' kernel/printk/printk_safe.c

/*
* Safe printk() for NMI context. It uses a per-CPU buffer to
* store the message. NMIs are not nested, so there is always only
* one writer running. But the buffer might get flushed from another
* CPU, so we need to be careful.
*/
static int vprintk_safe_nmi(const char *fmt, va_list args)

> > > > +#ifdef CONFIG_PRINTK_NMI
> > > > +/*
> > > > + * Safe printk() for NMI context. It uses a per-CPU buffer to
> > > > + * store the message. NMIs are not nested, so there is always only
> > > > + * one writer running. But the buffer might get flushed from another
> > > > + * CPU, so we need to be careful.
> > > > + */
> > >
> > > Hmm, I wanted to describe why we need another per-CPU buffer in NMI
> > > and I am not sure that we really need it.
> >
> > NMI-printk can interrupt safe-printk's vsnprintf() in the middle of
> > the "while (*fmt)" loop: safe-priNMI-PRINTK
>
> But this already happens when any of the WARNs is triggered
> inside vsnprintf(). Either this is safe or we are in
> trouble.

the point was that when printk-safe resumes after being interrupted
by NMI-printk it continues printing from the offset at which it has
been interrupted, writing over the lines that were sprintf-d by NMI
printk; because NMI-printk used the same buffer offset `s->len'. so
at least part of NMI-printk message will be lost.

-ss