Message-ID: <CAJZ5v0i6EgDxhziF4jEvsrAnRqaWO83z697VrRFQSEdPoCn4Qw@mail.gmail.com>
Date:	Fri, 18 Sep 2015 01:59:37 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Grygorii Strashko <grygorii.strashko@...com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Alan Stern <stern@...land.harvard.edu>,
	Thierry Reding <thierry.reding@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

Hi,

On Thu, Sep 17, 2015 at 5:48 PM, Grygorii Strashko
<grygorii.strashko@...com> wrote:
> Hi,
>
> On 09/17/2015 03:07 AM, Rafael J. Wysocki wrote:
>> On Wednesday, September 16, 2015 03:27:55 PM Alan Stern wrote:
>>> On Wed, 16 Sep 2015, Grygorii Strashko wrote:
>>>
>>>> I think, It should prohibited to probe devices during suspend/hibernation.
>>>> And solution introduced in this patch might help to fix it -
>>>> in general, we could do :
>>>> - add sync point on suspend enter: wait_for_device_probe() and
>>>> - prohibit probing: move all devices which will request probing into
>>>> deferred_probe list
>>>> - one suspend exit: allow probing and do driver_deferred_probe_trigger
>>>
>>> That could work; it's a good idea.
>>>
>>>> I'd like to mention here that this patch will work only
>>>> if dmp_list will be filled according device creation order ("parent<-child" dependencies)
>>>> *AND* according device's probing order ("supplier<-consumer").
>>>> So, if there is the case when Parent device can be probed AFTER its children
>>>> - it will not work, because "parent<-child" dependencies will not be tracked
>>>> any more :( Sry, I could not even imagine that such crazy case exist :'(
>>>
>>> If we avoid moving devices to the end of the dpm_list when they already
>>> have children, then we should be okay, right?
>>>
>>>> Are there any other subsystems with the same behavior like PCI?
>>>
>>> I don't know.
>>>
>>>> If not - probably, it could be fixed in PCI subsystem using device_pm_move_after() or
>>>> device_move() in PCIe ports probe.
>>>> if yes - ... maybe we can scan/re-check and reorder dpm_list on suspend enter and
>>>> restore ("parent<-child" dependencies).
>>>
>>>> Truth is that smth. need to be done 100%. Personally, I was hit by this issue also,
>>>>   and it cost me 3 hours of debugging and I came up with the same patch as
>>>> Bill Huang, then spent some time trying to understand what is wrong with PCI
>>>> - finally, I've just changed the order of my devices in DT :)
>>>>
>>>> Also, I think, it will be good to have this patch in -next to collect more feedbacks.
>>>
>>> I like the idea of forcing all probes during a sleep transition to be
>>> deferred.  We could carry them out just before unfreezing the user
>>> threads.  That combined with the change mentioned above ought to be
>>> worth testing.
>>
>> Agreed.
>>
>
> I've prepared code change which should prohibit devices probing during suspend/hibernation
> (below). It also expected to fix wait_for_device_probe() to take into account the case
> when the deferred probe workqueue could be still active.
>
> NOTE: It's only compile time tested!
>
> I'm very sorry that I'm replying here instead of sending a proper patch -
> I'm on business trip right now and I will be traveling next week also and will not
> be able to work on it intensively.
>
> If proposed approach is correct I can send RFC/RFT patch/es (or anyone else could
> pick up it if interested to move forward faster).
>
> --
> regards,
> -grygorii
>
> From d29e554bf1d593c6c52d2902872ba8a6c48a80a8 Mon Sep 17 00:00:00 2001
> From: Grygorii Strashko <grygorii.strashko@...com>
> Date: Thu, 17 Sep 2015 18:33:54 +0300
> Subject: [RFC/RFT PATCH] PM / sleep: prohibit devices probing during suspend/hibernation
>
> Signed-off-by: Grygorii Strashko <grygorii.strashko@...com>
> ---
>  drivers/base/dd.c      | 28 +++++++++++++++++++++++++++-
>  include/linux/device.h |  1 +
>  kernel/power/process.c |  8 ++++++++
>  3 files changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index be0eb46..dcadf30 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -55,6 +55,14 @@ static struct workqueue_struct *deferred_wq;
>  static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
>
>  /*
> + * In some cases, like suspend to RAM or hibernation, It might be reasonable
> + * to prohibit probing of devices as it could be unsafe.
> + * Once driver_force_probe_deferral is true all drivers probes will
> + * be forcibly deferred
> + */
> +static bool driver_force_probe_deferral;

What about defer_all_probes ?

> +
> +/*
>   * deferred_probe_work_func() - Retry probing devices in the active list.
>   */
>  static void deferred_probe_work_func(struct work_struct *work)
> @@ -171,6 +179,14 @@ static void driver_deferred_probe_trigger(void)
>         queue_work(deferred_wq, &deferred_probe_work);
>  }
>
> +void device_force_probe_deferral(bool enable)

device_defer_all_probes ?

> +{
> +       driver_force_probe_deferral = enable;
> +       if (!enable)
> +               driver_deferred_probe_trigger();
> +}
> +EXPORT_SYMBOL_GPL(device_force_probe_deferral);

That doesn't need to be exported, it is only called by statically linked code.

> +
>  /**
>   * deferred_probe_initcall() - Enable probing of deferred devices
>   *
> @@ -277,9 +293,15 @@ static DECLARE_WAIT_QUEUE_HEAD(probe_waitqueue);
>
>  static int really_probe(struct device *dev, struct device_driver *drv)
>  {
> -       int ret = 0;
> +       int ret = -EPROBE_DEFER;
>         int local_trigger_count = atomic_read(&deferred_trigger_count);
>
> +       if (driver_force_probe_deferral) {

What if the above is evaluated before the suspend sequence starts ->

> +               dev_dbg(dev, "Driver %s force probe deferral\n", drv->name);
> +               driver_deferred_probe_add(dev);
> +               return ret;
> +       }
> +

-> and the code below runs after it has started?

Isn't that racy?

>         atomic_inc(&probe_count);
>         pr_debug("bus: '%s': %s: probing driver %s with device %s\n",
>                  drv->bus->name, __func__, drv->name, dev_name(dev));
> @@ -391,6 +413,10 @@ int driver_probe_done(void)
>   */
>  void wait_for_device_probe(void)
>  {
> +       /* wait for the deferred probe workqueue to finish */
> +       if (driver_deferred_probe_enable)
> +               flush_workqueue(deferred_wq);
> +
>         /* wait for the known devices to complete their probing */
>         wait_event(probe_waitqueue, atomic_read(&probe_count) == 0);
>         async_synchronize_full();
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 5d7bc63..c68b8e1 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1034,6 +1034,7 @@ extern int  __must_check device_attach(struct device *dev);
>  extern int __must_check driver_attach(struct device_driver *drv);
>  extern void device_initial_probe(struct device *dev);
>  extern int __must_check device_reprobe(struct device *dev);
> +extern void device_force_probe_deferral(bool enable);
>
>  /*
>   * Easy functions for dynamically creating devices on the fly
> diff --git a/kernel/power/process.c b/kernel/power/process.c
> index 564f786..c13e78d 100644
> --- a/kernel/power/process.c
> +++ b/kernel/power/process.c
> @@ -148,6 +148,13 @@ int freeze_processes(void)
>         if (!error && !oom_killer_disable())
>                 error = -EBUSY;
>
> +       if (!error) {
> +               /** wait for the known devices to complete their probing */
> +               wait_for_device_probe();
> +               device_force_probe_deferral(true);
> +               wait_for_device_probe();

Ah, OK.  So the second wait_for_device_probe() avoids the race.

What is the first one for?

In any case, maybe call that from dpm_suspend_start() after
dpm_prepare() has run successfully?  This is the point we need to
start to block probing after all.

> +       }
> +
>         if (error)
>                 thaw_processes();
>         return error;
> @@ -190,6 +197,7 @@ void thaw_processes(void)
>                 atomic_dec(&system_freezing_cnt);
>         pm_freezing = false;
>         pm_nosig_freezing = false;
> +       device_force_probe_deferral(false);

And why don't you call that from dpm_resume_end()?

>
>         oom_killer_enable();
>
> --

Thanks,
Rafael
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