linux-kernel - Re: [PATCH 2/3] opp: Allocate the OPP table outside of opp_table

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Date:   Fri, 6 Nov 2020 12:56:24 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>, robdclark@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] opp: Allocate the OPP table outside of opp_table_lock

On 02-11-20, 10:51, Viresh Kumar wrote:
> diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> +/*
> + * We need to make sure that the OPP table for a device doesn't get added twice,
> + * if this routine gets called in parallel with the same device pointer.
> + *
> + * The simplest way to enforce that is to perform everything (find existing
> + * table and if not found, create a new one) under the opp_table_lock, so only
> + * one creator gets access to the same. But that expands the critical section
> + * under the lock and may end up causing circular dependencies with frameworks
> + * like debugfs, interconnect or clock framework as they may be direct or
> + * indirect users of OPP core.
> + *
> + * And for that reason we have to go for a bit tricky implementation here, which
> + * uses the opp_tables_busy flag to indicate if another creator is in the middle
> + * of adding an OPP table and others should wait for it to finish.
> + */
>  static struct opp_table *_opp_get_opp_table(struct device *dev, int index)
>  {
>  	struct opp_table *opp_table;
>  
> -	/* Hold our table modification lock here */
> +again:
>  	mutex_lock(&opp_table_lock);
>  
>  	opp_table = _find_opp_table_unlocked(dev);
>  	if (!IS_ERR(opp_table))
>  		goto unlock;
>  
> +	/*
> +	 * The opp_tables list or an OPP table's dev_list is getting updated by
> +	 * another user, wait for it to finish.
> +	 */
> +	if (unlikely(opp_tables_busy)) {
> +		mutex_unlock(&opp_table_lock);
> +		cpu_relax();
> +		goto again;
> +	}
> +
> +	opp_tables_busy = true;
>  	opp_table = _managed_opp(dev, index);
> +
> +	/* Drop the lock to reduce the size of critical section */
> +	mutex_unlock(&opp_table_lock);
> +
>  	if (opp_table) {
>  		if (!_add_opp_dev(dev, opp_table)) {
>  			dev_pm_opp_put_opp_table(opp_table);
>  			opp_table = ERR_PTR(-ENOMEM);
>  		}
> -		goto unlock;
> +
> +		mutex_lock(&opp_table_lock);
> +	} else {
> +		opp_table = _allocate_opp_table(dev, index);
> +
> +		mutex_lock(&opp_table_lock);
> +		if (!IS_ERR(opp_table))
> +			list_add(&opp_table->node, &opp_tables);
>  	}
>  
> -	opp_table = _allocate_opp_table(dev, index);
> +	opp_tables_busy = false;

And here is a fix that will be merged with this patch while applying.  It is
required as _allocate_opp_table() (which calls _find_table_of_opp_np())  isn't
called with the opp_table_lock anymore.

diff --git a/drivers/opp/of.c b/drivers/opp/of.c
index c718092757d9..6b7f0066942d 100644
--- a/drivers/opp/of.c
+++ b/drivers/opp/of.c
@@ -112,8 +112,6 @@ static struct opp_table *_find_table_of_opp_np(struct device_node *opp_np)
        struct opp_table *opp_table;
        struct device_node *opp_table_np;
 
-       lockdep_assert_held(&opp_table_lock);
-
        opp_table_np = of_get_parent(opp_np);
        if (!opp_table_np)
                goto err;
@@ -121,12 +119,15 @@ static struct opp_table *_find_table_of_opp_np(struct device_node *opp_np)
        /* It is safe to put the node now as all we need now is its address */
        of_node_put(opp_table_np);
 
+       mutex_lock(&opp_table_lock);
        list_for_each_entry(opp_table, &opp_tables, node) {
                if (opp_table_np == opp_table->np) {
                        _get_opp_table_kref(opp_table);
+                       mutex_unlock(&opp_table_lock);
                        return opp_table;
                }
        }
+       mutex_unlock(&opp_table_lock);
 
 err:
        return ERR_PTR(-ENODEV);

-- 
viresh