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SubjectRe: [PATCH 2/4 v5] memstick: Prevent memstick host from getting runtime suspended during card detection
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> On Oct 29, 2018, at 20:25, Ulf Hansson <ulf.hansson@linaro.org> wrote:
>
> On 24 October 2018 at 10:49, Kai-Heng Feng <kai.heng.feng@canonical.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@canonical.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.

Or is there a better way in your mind?

Kai-Heng

>
> Kind regards
> Uffe

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