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Date: Wed, 4 Feb 2015 20:48:38 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Stephen Boyd <sboyd@codeaurora.org>
Cc: Daniel Thompson <daniel.thompson@linaro.org>,
        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>,
        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: <20150204204838.65411577@grimm.local.home>
In-Reply-To: <20150205012351.GC30372@codeaurora.org>
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On Wed, 4 Feb 2015 17:23:51 -0800
Stephen Boyd <sboyd@codeaurora.org> wrote:

> 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?

Why? Isn't this for update, not for readers?

-- Steve

> 
> > +{
> > +	/* 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);
> > +}
> > +
> > +/*
> > 
>