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

From: Peter Zijlstra <peterz@infradead.org>
To: "Suresh E. Warrier" <warrier@linux.vnet.ibm.com>
Cc: lklml <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@redhat.com>, Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 2/2] More precise timestamps for nested writes
Date: Tue, 14 Apr 2015 19:13:24 +0200	[thread overview]
Message-ID: <20150414171324.GE17717@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <552C7D89.2080802@linux.vnet.ibm.com>

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;
> +	}
> +}