linux-kernel - Re: [RFC][PATCHv5 3/7] printk: introduce per-cpu safe

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Date:   Mon, 12 Dec 2016 23:12:30 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv5 3/7] printk: introduce per-cpu safe_print seq
 buffer

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