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

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Date:   Tue, 30 Oct 2018 14:03:08 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Kai Heng Feng <kai.heng.feng@...onical.com>
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 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.

>
> 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?

Kind regards
Uffe