linux-kernel - Re: [RFC PATCH 2/7] printk: Simple implementation for NMI backtracing

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Message-ID: <20150319183037.GT23123@twins.programming.kicks-ass.net>
Date:	Thu, 19 Mar 2015 19:30:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Andrew Thoelke <andrew.thoelke@....com>
Subject: Re: [RFC PATCH 2/7] printk: Simple implementation for NMI backtracing

On Thu, Mar 19, 2015 at 01:39:58PM -0400, Steven Rostedt wrote:
> > +void printk_nmi_backtrace_complete(void)
> > +{
> > +	struct nmi_seq_buf *s;
> > +	int len, cpu, i, last_i;
> > +
> > +	/*
> > +	 * Now that all the NMIs have triggered, we can dump out their
> > +	 * back traces safely to the console.
> > +	 */
> > +	for_each_possible_cpu(cpu) {
> > +		s = &per_cpu(nmi_print_seq, cpu);
> > +		last_i = 0;
> > +
> > +		len = seq_buf_used(&s->seq);
> > +		if (!len)
> > +			continue;
> > +
> > +		/* Print line by line. */
> > +		for (i = 0; i < len; i++) {
> > +			if (s->buffer[i] == '\n') {
> > +				print_seq_line(s, last_i, i);
> > +				last_i = i + 1;
> > +			}
> > +		}
> > +		/* Check if there was a partial line. */
> > +		if (last_i < len) {
> > +			print_seq_line(s, last_i, len - 1);
> > +			pr_cont("\n");
> > +		}
> > +
> > +		/* Wipe out the buffer ready for the next time around. */
> > +		seq_buf_clear(&s->seq);
> > +	}
> > +
> > +	clear_bit(0, &nmi_print_flag);
> > +	smp_mb__after_atomic();
> 
> Is this really necessary. What is the mb synchronizing?
> 
> [ Added Peter Zijlstra to confirm it's not needed ]

It surely looks suspect; and it lacks a comment, which is a clear sign
its buggy.

Now it if tries to order the accesses to the seqbuf againt the clearing
of the bit one would have expected a _before_ barrier, not an _after_.

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