linux-kernel - Re: [PATCH 2/2] More precise timestamps for nested writes

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Message-ID: <20150414171324.GE17717@twins.programming.kicks-ass.net>
Date:	Tue, 14 Apr 2015 19:13:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Suresh E. Warrier" <warrier@...ux.vnet.ibm.com>
Cc:	lklml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 2/2] More precise timestamps for nested writes

On Mon, Apr 13, 2015 at 09:38:01PM -0500, Suresh E. Warrier wrote:
> +static u64 *get_write_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
> +				unsigned long *flags)
> +{
> +	if (rb_precise_nested_write_ts()) {
> +		/*
> +		 * Ensure that we are not preempted until after we update
> +		 * the write timestamp.
> +		 */
> +		local_irq_save(*flags);
> +		return &cpu_buffer->last_stamp;

Yeah, ever hear about NMIs? This isn't going to work.

> +	} else {
> +		return &cpu_buffer->write_stamp;
> +	}
> +}
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