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

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Message-ID: <20161210031022.GA493@tigerII.localdomain>
Date:   Sat, 10 Dec 2016 12:10:22 +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 (12/09/16 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.


> >  	 * Make sure that all old data have been read before the buffer was
> > @@ -261,14 +263,95 @@ void printk_safe_flush_on_panic(void)
> >  	printk_safe_flush();
> >  }
> >  
> > +#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


	-ss