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Date: Wed, 4 Feb 2015 17:23:51 -0800
From: Stephen Boyd <sboyd@codeaurora.org>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, John Stultz <john.stultz@linaro.org>,
        linux-kernel@vger.kernel.org, patches@linaro.org,
        linaro-kernel@lists.linaro.org, Sumit Semwal <sumit.semwal@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Russell King <linux@arm.linux.org.uk>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v3 4/4] sched_clock: Avoid deadlock during read from NMI
Message-ID: <20150205012351.GC30372@codeaurora.org>
References: <1421859236-19782-1-git-send-email-daniel.thompson@linaro.org>
 <1422644602-11953-1-git-send-email-daniel.thompson@linaro.org>
 <1422644602-11953-5-git-send-email-daniel.thompson@linaro.org>
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On 01/30, Daniel Thompson wrote:
> @@ -98,26 +98,50 @@ unsigned long long notrace sched_clock(void)
>  }
>  
>  /*
> + * Updating the data required to read the clock.
> + *
> + * sched_clock will never observe mis-matched data even if called from
> + * an NMI. We do this by maintaining an odd/even copy of the data and
> + * steering sched_clock to one or the other using a sequence counter.
> + * In order to preserve the data cache profile of sched_clock as much
> + * as possible the system reverts back to the even copy when the update
> + * completes; the odd copy is used *only* during an update.
> + */
> +static void update_clock_read_data(struct clock_read_data *rd)

notrace?

> +{
> +	/* update the backup (odd) copy with the new data */
> +	cd.read_data[1] = *rd;
> +
> +	/* steer readers towards the new data */

s/new data/odd copy/?

> +	raw_write_seqcount_latch(&cd.seq);
> +
> +	/* now its safe for us to update the normal (even) copy */
> +	cd.read_data[0] = *rd;
> +
> +	/* switch readers back to the even copy */
> +	raw_write_seqcount_latch(&cd.seq);
> +}
> +
> +/*
> 

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