linux-kernel - Re: [PATCH v2] printk: Use the main logbuf in NMI when logbuf

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Date:   Wed, 17 May 2017 17:08:17 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Russell King <rmk+kernel@....linux.org.uk>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Metcalf <cmetcalf@...hip.com>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v2] printk: Use the main logbuf in NMI when logbuf_lock
 is available

On Fri 2017-05-05 11:21:41, Sergey Senozhatsky wrote:
> On (05/04/17 17:46), Petr Mladek wrote:
> [..]
> > Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> 
> Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

Thanks for ack.

> a small nitpick,
> 
> [..]
> >  void printk_nmi_enter(void)
> >  {
> > -	this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> > +	/*
> > +	 * The size of the extra per-CPU buffer is limited. Use it only when
> > +	 * the main one is locked. If this CPU is not in the safe context,
> > +	 * the lock must be taken on another CPU and we could wait for it.
> > +	 */
> > +	if (raw_spin_is_locked(&logbuf_lock) &&
> > +	    this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) {
> > +		this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> 
> could we please check `printk_context' (local to a particular CPU)
> first and, if positive, then access `logbuf_lock' (which is global)?

OK, I did so and queued the fix for-4.13, see
https://git.kernel.org/pub/scm/linux/kernel/git/pmladek/printk.git/commit/?h=for-4.13&id=ad30113010b7b2dd886923261aea7034de07cce6

Best Regards,
Petr