linux-kernel - Re: [PATCH 2/4 v5] memstick: Prevent memstick host from getting runtime suspended during card detection

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Message-Id: <39EAB186-BE6B-4CB2-B9EE-88B0C267888A@canonical.com>
Date:   Wed, 31 Oct 2018 14:33:32 +0800
From:   Kai Heng Feng <kai.heng.feng@...onical.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Linux USB List <linux-usb@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4 v5] memstick: Prevent memstick host from getting
 runtime suspended during card detection



> On Oct 31, 2018, at 12:04 AM, Ulf Hansson <ulf.hansson@...aro.org> wrote:
> 
> On 30 October 2018 at 16:23, Kai Heng Feng <kai.heng.feng@...onical.com> wrote:
>> 
>> 
>>> On Oct 30, 2018, at 21:03, Ulf Hansson <ulf.hansson@...aro.org> wrote:
>>> 
>>> On 29 October 2018 at 17:31, Kai Heng Feng <kai.heng.feng@...onical.com> wrote:
>>>> 
>>>> 
>>>>> On Oct 29, 2018, at 20:25, Ulf Hansson <ulf.hansson@...aro.org> wrote:
>>>>> 
>>>>> On 24 October 2018 at 10:49, Kai-Heng Feng <kai.heng.feng@...onical.com> wrote:
>>>>>> We can use MEMSTICK_POWER_{ON,OFF} along with pm_runtime_{get,put}
>>>>>> helpers to let memstick host support runtime pm.
>>>>>> 
>>>>>> There's a small window between memstick_detect_change() and its queued
>>>>>> work, memstick_check(). In this window the rpm count may go down to zero
>>>>>> before the memstick host powers on, so the host can be inadvertently
>>>>>> suspended.
>>>>>> 
>>>>>> Increment rpm count before calling memstick_check(), and decrement rpm
>>>>>> count afterward, as now we are sure the memstick host should be
>>>>>> suspended or not.
>>>>>> 
>>>>>> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
>>>>>> ---
>>>>>> drivers/memstick/core/memstick.c | 4 ++++
>>>>>> 1 file changed, 4 insertions(+)
>>>>>> 
>>>>>> diff --git a/drivers/memstick/core/memstick.c b/drivers/memstick/core/memstick.c
>>>>>> index 76382c858c35..5f16a8826401 100644
>>>>>> --- a/drivers/memstick/core/memstick.c
>>>>>> +++ b/drivers/memstick/core/memstick.c
>>>>>> @@ -18,6 +18,7 @@
>>>>>> #include <linux/delay.h>
>>>>>> #include <linux/slab.h>
>>>>>> #include <linux/module.h>
>>>>>> +#include <linux/pm_runtime.h>
>>>>>> 
>>>>>> #define DRIVER_NAME "memstick"
>>>>>> 
>>>>>> @@ -209,6 +210,7 @@ static int memstick_dummy_check(struct memstick_dev *card)
>>>>>> */
>>>>>> void memstick_detect_change(struct memstick_host *host)
>>>>>> {
>>>>>> +       pm_runtime_get_noresume(host->dev.parent);
>>>>>>      queue_work(workqueue, &host->media_checker);
>>>>>> }
>>>>>> EXPORT_SYMBOL(memstick_detect_change);
>>>>>> @@ -479,6 +481,8 @@ static void memstick_check(struct work_struct *work)
>>>>>>              host->set_param(host, MEMSTICK_POWER, MEMSTICK_POWER_OFF);
>>>>>> 
>>>>>>      mutex_unlock(&host->lock);
>>>>>> +
>>>>>> +       pm_runtime_put(host->dev.parent);
>>>>>>      dev_dbg(&host->dev, "memstick_check finished\n");
>>>>>> }
>>>>>> 
>>>>> 
>>>>> I am not sure this works, sorry.
>>>>> 
>>>>> More precisely, I don't think there is a guarantee that the calls to
>>>>> pm_runtime_get|put*() becomes properly balanced. In principle
>>>>> memstick_detect_change() could be called, without actually causing a
>>>>> new work to be scheduled if there is already such a work in the queue
>>>>> (depends on the workqueue configuration). Isn't it so?
>>>> 
>>>> You are right.
>>>> 
>>>> We can use test_and_set_bit() or alike to properly balance pm_runtime
>>>> helpers, but the most straightforward solution in my mind is to merge
>>>> memstick_detect_change() and memstick_check() as one function.
>>>> 
>>>> memstick_detect_change() it’s the only user of memstick_check() anyway.
>>> 
>>> I suspect memstick_detect_change() is supposed to be called by host
>>> drivers, when they receive some kind of notification due to a card
>>> being inserted or removed. I guess that happen (at least
>>> hypothetically) also from atomic (IRQ) context.
>>> 
>>> As memstick_check() is doing hole bunch of operations, I am not sure
>>> bypassing the work-queue is a good idea, if that is what you are
>>> proposing.
>> 
>> Okay, it’s better to keep it that way.
>> 
>>> 
>>>> 
>>>> Or is there a better way in your mind?
>>> 
>>> I don't know.
>>> 
>>> Well, I am not sure I understand why you need to call
>>> pm_runtime_get_noresume() from memstick_detect_change() in the first
>>> place. Could you explain that in more detail?
>> 
>> I guess it didn’t explain it well enough in the log, let me add some detail:
>> There's a small window between memstick_detect_change() and its queued
>> work, memstick_check(). In this window the rpm count may go down to zero
>> before the memstick host powers on, where I use
>> pm_runtime_get_noresume() to increment the rpm count.
>> 
>> memstick_check() uses some functions in rtsx_usb_ms that have
>> pm_runtime_put*() so the rpm count may go down to zero, before the
>> memstick host powers on.
> 
> So then, why doesn't memstick_check() early on calls
> pm_runtime_get_sync() and when it has finished with probing for a
> card, balance that with a call pm_runtime_put()?

This will do, not sure what I was thinking. Thanks for pointing out.

Kai-Heng

> 
> Kind regards
> Uffe