linux-kernel - Re: [PATCH V2 2/3] PM / OPP: Parse 'opp-supported-hw' binding

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Date:	Tue, 1 Dec 2015 12:22:52 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>, nm@...com,
	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Dmitry Torokhov <dtor@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Len Brown <len.brown@...el.com>,
	open list <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>, Shawn Guo <shawnguo@...nel.org>
Subject: Re: [PATCH V2 2/3] PM / OPP: Parse 'opp-supported-hw' binding

On 27-11-15, 10:15, Viresh Kumar wrote:
> > > +	dev_opp->supported_hw = kmemdup(versions, count * sizeof(*versions),
> > > +					GFP_KERNEL);
> > 
> > And then we're going to modify said opp here under the mutex
> > lock.
> 
> opp-dev ..
> 
> > > +	if (!dev_opp->supported_hw) {
> > > +		ret = -ENOMEM;
> > > +		goto err;
> > > +	}
> > > +
> > > +	dev_opp->supported_hw_count = count;
> > 
> > So we've properly handled the concurrent writer case (which is
> > probably not even real), but we have improperly handled the case
> > where a reader is running in parallel to the writer. We should
> > only list_add_rcu the pointer once we're done modifying the
> > pointer we created. Otherwise a reader can come along and see the
> > half initialized structure, which is not good.
> 
> This function will be called, from some platform code, before the OPP
> table is initialized. It isn't useful to call it after the OPPs are
> added for the device. So there wouldn't be any concurrent reader.

Since these functions are *only* going to be called before any OPPs
are added for the device, and hence ruling out any concurrent readers,
maybe we can guarantee that with this:

diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
index 5449bae74a44..ec74d98afe75 100644
--- a/drivers/base/power/opp/core.c
+++ b/drivers/base/power/opp/core.c
@@ -876,6 +876,9 @@ int dev_pm_opp_set_supported_hw(struct device *dev, const u32 *versions,
                goto err;
        }
 
+       /* Make sure there are no concurrent readers while updating dev_opp */
+       WARN_ON(!list_empty(&dev_opp->opp_list));
+
        dev_opp->supported_hw = kmemdup(versions, count * sizeof(*versions),
                                        GFP_KERNEL);
        if (!dev_opp->supported_hw) {
@@ -924,6 +927,9 @@ void dev_pm_opp_put_supported_hw(struct device *dev)
                goto unlock;
        }
 
+       /* Make sure there are no concurrent readers while updating dev_opp */
+       WARN_ON(!list_empty(&dev_opp->opp_list));
+
        if (!dev_opp->supported_hw) {
                dev_err(dev, "%s: Doesn't have supported hardware list\n",
                        __func__);


I don't really want to create a duplicate dev_opp here and then
replace that in the list, because we know that we have just created
it.

Over that, if a reference to dev_opp is used somewhere else, lets say
within the pm_opp structure, then updating all OPPs at such times
would be really hard.

Lets close this before you go for vacations. I will get whatever
solution you feel is right.

-- 
viresh
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