Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Ulf,

On Tue, Jun 23, 2015 at 2:50 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote:
> On 22 June 2015 at 09:31, Geert Uytterhoeven <geert+renesas@glider.be> wrote:
>> If pm_genpd_{add,remove}_device() keeps on failing with -EAGAIN, we end
>> up with an infinite loop in genpd_dev_pm_{at,de}tach().
>>
>> This may happen due to a genpd.prepared_count imbalance.  This is a bug
>> elsewhere, but it will result in a system lock up, possibly during
>> reboot of an otherwise functioning system.
>>
>> To avoid this, put a limit on the maximum number of loop iterations,
>> including a simple back-off mechanism.  If the limit is reached, the
>> operation will just fail.  An error message is already printed.
>>
>> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
>> ---
>>  drivers/base/power/domain.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
>> index cdd547bd67df8218..60e0309dd8dd0264 100644
>> --- a/drivers/base/power/domain.c
>> +++ b/drivers/base/power/domain.c
>> @@ -6,6 +6,7 @@
>>   * This file is released under the GPLv2.
>>   */
>>
>> +#include <linux/delay.h>
>>  #include <linux/kernel.h>
>>  #include <linux/io.h>
>>  #include <linux/platform_device.h>
>> @@ -19,6 +20,9 @@
>>  #include <linux/suspend.h>
>>  #include <linux/export.h>
>>
>> +#define GENPD_RETRIES  20
>> +#define GENPD_DELAY_US 10
>> +
>>  #define GENPD_DEV_CALLBACK(genpd, type, callback, dev)         \
>>  ({                                                             \
>>         type (*__routine)(struct device *__d);                  \
>> @@ -2131,6 +2135,7 @@ EXPORT_SYMBOL_GPL(of_genpd_get_from_provider);
>>  static void genpd_dev_pm_detach(struct device *dev, bool power_off)
>>  {
>>         struct generic_pm_domain *pd;
>> +       unsigned int i;
>>         int ret = 0;
>>
>>         pd = pm_genpd_lookup_dev(dev);
>> @@ -2139,10 +2144,13 @@ static void genpd_dev_pm_detach(struct device *dev, bool power_off)
>>
>>         dev_dbg(dev, "removing from PM domain %s\n", pd->name);
>>
>> -       while (1) {
>> +       for (i = 0; i < GENPD_RETRIES; i++) {
>>                 ret = pm_genpd_remove_device(pd, dev);
>>                 if (ret != -EAGAIN)
>>                         break;
>> +
>> +               if (i > GENPD_RETRIES / 2)
>> +                       udelay(GENPD_DELAY_US);
>>                 cond_resched();
>>         }
>>
>> @@ -2183,6 +2191,7 @@ int genpd_dev_pm_attach(struct device *dev)
>>  {
>>         struct of_phandle_args pd_args;
>>         struct generic_pm_domain *pd;
>> +       unsigned int i;
>>         int ret;
>>
>>         if (!dev->of_node)
>> @@ -2218,10 +2227,13 @@ int genpd_dev_pm_attach(struct device *dev)
>>
>>         dev_dbg(dev, "adding to PM domain %s\n", pd->name);
>>
>> -       while (1) {
>> +       for (i = 0; i < GENPD_RETRIES; i++) {
>>                 ret = pm_genpd_add_device(pd, dev);
>>                 if (ret != -EAGAIN)
>>                         break;
>> +
>> +               if (i > GENPD_RETRIES / 2)
>> +                       udelay(GENPD_DELAY_US);
>
> In this execution path, we retry when getting -EAGAIN while believing
> the reason to the error are only *temporary* as we are soon waiting
> for all devices in the genpd to be system PM resumed. At least that's
> my understanding to why we want to deal with -EAGAIN here, but I might
> be wrong.
>
> In this regards, I wonder whether it could be better to re-try only a
> few times but with a far longer interval time than a couple us. What
> do you think?

That's indeed viable. I have no idea for how long this temporary state can
extend.

> However, what if the reason to why we get -EAGAIN isn't *temporary*,
> because we are about to enter system PM suspend state. Then the caller
> of this function which comes via some bus' ->probe(), will hang until
> the a system PM resume is completed. Is that really going to work? So,
> for this case your limited re-try approach will affect this scenario
> as well, have you considered that?

There's a limit on the number of retries, so it won't hang indefinitely.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds