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

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Message-Id: <E8BC3191-E48C-423C-B704-929F7FAC4930@canonical.com>
Date:   Tue, 30 Oct 2018 00:31:41 +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 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.

Or is there a better way in your mind?

Kai-Heng

> 
> Kind regards
> Uffe