linux-kernel - Re: [PATCH] opp: Invalidate current opp when draining the opp list

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Date:   Fri, 5 Mar 2021 09:54:01 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Beata Michalska <beata.michalska@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        vireshk@...nel.org, nm@...com, sboyd@...nel.org
Subject: Re: [PATCH] opp: Invalidate current opp when draining the opp list

On 04-03-21, 15:07, Beata Michalska wrote:
> The current_opp when set, grabs additional reference on the opp,
> which is then supposed to be dropped upon releasing the opp table.
> Still both dev_pm_opp_remove_table and dev_pm_opp_remove_all_dynamic
> will completely drain the OPPs list, including dropping the additional
> reference on current_opp. This may lead to an attempt to access
> memory that has already been released. Make sure that while draining
> the list (in both dynamic and static cases) the current_opp gets
> actually invalidated.
> 
> Fixes: 81c4d8a3c414 ("opp: Keep track of currently programmed OPP")
> 
> Signed-off-by: Beata Michalska <beata.michalska@....com>
> ---
>  drivers/opp/core.c | 49 ++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 32 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> index c268938..10e65c4 100644
> --- a/drivers/opp/core.c
> +++ b/drivers/opp/core.c
> @@ -1502,10 +1502,39 @@ static struct dev_pm_opp *_opp_get_next(struct opp_table *opp_table,
>  	return opp;
>  }
>  
> -bool _opp_remove_all_static(struct opp_table *opp_table)
> +static int __opp_drain_list(struct opp_table *opp_table, bool dynamic)
>  {
>  	struct dev_pm_opp *opp;
> +	int count = 0;
> +
> +	/*
> +	 * Can't remove the OPP from under the lock, debugfs removal needs to
> +	 * happen lock less to avoid circular dependency issues.
> +	 */
> +	while ((opp = _opp_get_next(opp_table, dynamic))) {
> +		/*
> +		 * The current_opp has extra hold on the ref count,
> +		 * still the draining here will result in all of them
> +		 * being dropped completely, so make
> +		 * sure no one will try to access the current_opp
> +		 * afterwords
> +		 */
> +		if (opp_table->current_opp == opp &&
> +		    !(kref_read(&opp->kref) - 1))
> +			opp_table->current_opp = NULL;

Did you miss looking at:

static void _opp_table_kref_release(struct kref *kref)
{
        ...

	if (opp_table->current_opp)
		dev_pm_opp_put(opp_table->current_opp);

        ...
}

?

This is the place where the last reference to the current_opp is released and so
we shouldn't have any invalid access to it anywhere else.

Or am I missing some context here ?

-- 
viresh