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Date: Mon, 21 Oct 2019 07:55:16 +0530 From: Viresh Kumar <viresh.kumar@...aro.org> To: Stephen Boyd <sboyd@...nel.org> Cc: Nishanth Menon <nm@...com>, Viresh Kumar <vireshk@...nel.org>, linux-pm@...r.kernel.org, Vincent Guittot <vincent.guittot@...aro.org>, Rafael Wysocki <rjw@...ysocki.net>, Dmitry Osipenko <digetx@...il.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] opp: Reinitialize the list_kref before adding the static OPPs again On 18-10-19, 14:12, Stephen Boyd wrote: > Quoting Viresh Kumar (2019-10-18 02:28:41) > > The list_kref reaches a count of 0 when all the static OPPs are removed, > > for example when dev_pm_opp_of_cpumask_remove_table() is called, though > > the actual OPP table may not get freed as it may still be referenced by > > other parts of the kernel, like from a call to > > dev_pm_opp_set_supported_hw(). And if we call > > dev_pm_opp_of_cpumask_add_table() again at this point, we must > > reinitialize the list_kref otherwise the kernel will hit a WARN() in > > kref infrastructure for incrementing a kref with value 0. > > > > Fixes: 11e1a1648298 ("opp: Don't decrement uninitialized list_kref") > > Reported-by: Dmitry Osipenko <digetx@...il.com> > > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org> > > --- > > drivers/opp/of.c | 7 +++++++ > > 1 file changed, 7 insertions(+) > > > > diff --git a/drivers/opp/of.c b/drivers/opp/of.c > > index 6dc41faf74b5..1cbb58240b80 100644 > > --- a/drivers/opp/of.c > > +++ b/drivers/opp/of.c > > @@ -663,6 +663,13 @@ static int _of_add_opp_table_v2(struct device *dev, struct opp_table *opp_table) > > return 0; > > } > > > > + /* > > + * Re-initialize list_kref every time we add static OPPs to the OPP > > + * table as the reference count may be 0 after the last tie static OPPs > > s/tie/time/ > > > + * were removed. > > + */ > > + kref_init(&opp_table->list_kref); > > It seems racy. I am not sure if I see a race here, but maybe I am missing something. Care to explain ? > Why are we doing this vs. making an entirely new and > different OPP structure? Or why is the count reaching 0 when something > is obviously still referencing it? The kref for the opp table is opp_table->kref and the one here is different. This is list_kref which is used for freeing OPPs added statically from the DT. The static OPPs get added to the OPP table when one calls dev_pm_opp_of_cpumask_add_table() and must be removed on a call to dev_pm_opp_of_cpumask_remove_table(). The opp table structure may not get freed at this moment though as it is still referenced by the caller of dev_pm_opp_set_supported_hw(). And now when we try to add the static OPPs again (re-insertion of cpufreq module), we need to reinitialize the list_kref again as its count reached 0 earlier and the resources (static OPPs) were freed. -- viresh
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