linux-kernel - Re: [PATCH v3 4/4] sched_clock: Avoid deadlock during read from NMI

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Date:	Wed, 4 Feb 2015 20:48:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Russell King <linux@....linux.org.uk>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v3 4/4] sched_clock: Avoid deadlock during read from NMI

On Wed, 4 Feb 2015 17:23:51 -0800
Stephen Boyd <sboyd@...eaurora.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);
> > +}
> > +
> > +/*
> > 
> 

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