Re: [PATCH V2 5/8] PM / OPP: Add infrastructure to manage multiple regulators

Stephen Boyd
2016-11-10T01:37:04Z

On 10/26, Viresh Kumar wrote:
> On 25-10-16, 09:49, Stephen Boyd wrote:
> > On 10/20, Viresh Kumar wrote:
> > > diff --git a/drivers/base/power/opp/core.c b/drivers/base/power/opp/core.c
> > > index 37fad2eb0f47..45c70ce07864 100644
> > > --- a/drivers/base/power/opp/core.c
> > > +++ b/drivers/base/power/opp/core.c
> > > @@ -235,21 +237,44 @@ unsigned long dev_pm_opp_get_max_volt_latency(struct device *dev)
> > > return 0;
> > > }
> > >
> > > - reg = opp_table->regulator;
> > > - if (IS_ERR(reg)) {
> > > + count = opp_table->regulator_count;
> > > +
> > > + if (!count) {
> > > /* Regulator may not be required for device */
> > > rcu_read_unlock();
> > > return 0;
> > > }
> > >
> > > - list_for_each_entry_rcu(opp, &opp_table->opp_list, node) {
> > > - if (!opp->available)
> > > - continue;
> > > + size = count * sizeof(*regulators);
> > > + regulators = kmemdup(opp_table->regulators, size, GFP_KERNEL);
> >
> > How do we allocate under RCU? Doesn't that spit out big warnings?
>
> That doesn't. I even tried enabling several locking debug config options.

Please read RCU documentation. From rcu_read_lock() function
documentation:

In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
it is illegal to block while in an RCU read-side critical section.
In preemptible RCU implementations (TREE_PREEMPT_RCU) in CONFIG_PREEMPT
kernel builds, RCU read-side critical sections may be preempted,
but explicit blocking is illegal. Finally, in preemptible RCU
implementations in real-time (with -rt patchset) kernel builds, RCU
read-side critical sections may be preempted and they may also block, but
only when acquiring spinlocks that are subject to priority inheritance.

I suppose that in certain configurations it will warn and in
others it won't. I thought lockdep would always complain though,
so that's sad it doesn't. At least in some implementations of RCU
rcu_read_lock() is the same as preempt_disable(), which basically
means no sleeping calls like GFP_KERNEL allocations.

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