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

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Message-Id: <3881A95A-CDDE-4C69-859F-C1F92F0C5724@canonical.com>
Date:   Tue, 30 Oct 2018 23:23:59 +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 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.

Kai-Heng

> 
> Kind regards
> Uffe