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Date: Thu, 19 Mar 2015 19:30:37 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>,
        linux-arm-kernel@lists.infradead.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
        patches@linaro.org, linaro-kernel@lists.linaro.org,
        John Stultz <john.stultz@linaro.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Andrew Thoelke <andrew.thoelke@arm.com>
Subject: Re: [RFC PATCH 2/7] printk: Simple implementation for NMI backtracing
Message-ID: <20150319183037.GT23123@twins.programming.kicks-ass.net>
References: <1426688428-3150-1-git-send-email-daniel.thompson@linaro.org>
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 <20150319133958.4821493e@gandalf.local.home>
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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_.