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Subject: Re: [PATCH v10 54/55] Input: atmel_mxt_ts: Implement synchronization
 during various operation
From:   Dmitry Osipenko <digetx@gmail.com>
To:     "Wang, Jiada" <jiada_wang@mentor.com>, nick@shmanahar.org,
        dmitry.torokhov@gmail.com, jikos@kernel.org,
        benjamin.tissoires@redhat.com, bsz@semihalf.com
Cc:     linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
        erosca@de.adit-jv.com, Andrew_Gabbasov@mentor.com
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 <b06c7915-562f-ec68-766a-2118cfe57a0f@mentor.com>
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Message-ID: <638b79b3-30f0-31a0-aad8-55e3ea207ac7@gmail.com>
Date:   Thu, 2 Apr 2020 17:07:57 +0300
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02.04.2020 17:00, Dmitry Osipenko пишет:
> 02.04.2020 16:24, Dmitry Osipenko пишет:
>> 02.04.2020 14:50, Wang, Jiada пишет:
>>> Hi Dmitry
>>>
>>> On 2020/04/02 1:04, Dmitry Osipenko wrote:
>>>> 31.03.2020 13:50, Jiada Wang пишет:
>>>>> From: Sanjeev Chugh <sanjeev_chugh@mentor.com>
>>>>>
>>>>> There could be scope of race conditions when sysfs is being handled
>>>>> and at the same time, device removal is occurring. For example,
>>>>> we don't want the device removal to begin if the Atmel device
>>>>> cfg update is going on or firmware update is going on. In such
>>>>> cases, wait for device update to be completed before the removal
>>>>> continues.
>>>>>
>>>>>      Thread                                          Thread 2:
>>>>> =========================                      
>>>>> =========================
>>>>> mxt_update_fw_store()                           mxt_remove()
>>>>> mutex_lock(&data->lock)                         ...
>>>>> mxt_initialize()                                //Tries to acquire lock
>>>>>    request_firmware_nowait()                     mutex_lock(&data->lock)
>>>>> ...                                             ==>waits for lock()
>>>>> ...                                             .
>>>>> ...                                             .
>>>>> mutex_unlock(&data->lock)                       .
>>>>>                                                  //Gets lock and
>>>>> proceeds
>>>>>                                                 
>>>>> mxt_free_input_device();
>>>>>                                                  ...
>>>>>                                                 
>>>>> mutex_unlock(&data->lock)
>>>>>                                                  //Frees atmel driver
>>>>> data
>>>>>                                                  kfree(data)
>>>>>
>>>>> If the request_firmware_nowait() completes after the driver removal,
>>>>> and callback is triggered. But kernel crashes since the module is
>>>>> already removed.
>>>>>
>>>>> This commit adds state machine to serialize such scenarios.
>>>>
>>>> Won't it be easier to bump driver's module use-count by __module_get()
>>>> while firmware is updating? Or remove sysfs during of mxt_remove()? >
>>>
>>> thanks for your inspiration, I will replace state machine with module
>>> use-count.
>>
>> I'm actually now thinking that the suggestion about the module-count
>> wasn't very correct because this won't really help in regards to
>> mxt_update_fw_store() / mxt_remove() racing.
>>
>> I see that mxt_remove() already invokes the mxt_sysfs_remove(), which
>> should block until mxt_update_fw_store() is completed, shouldn't it?
>>
>> I guess the kfree(data) isn't the real cause of the problem and
>> something like this should help:
>>
>> diff --git a/drivers/input/touchscreen/atmel_mxt_ts.c
>> b/drivers/input/touchscreen/atmel_mxt_ts.c
>> index b2edf51e1595..4e66106feeb9 100644
>> --- a/drivers/input/touchscreen/atmel_mxt_ts.c
>> +++ b/drivers/input/touchscreen/atmel_mxt_ts.c
>> @@ -4254,6 +4254,7 @@ static void mxt_sysfs_remove(struct mxt_data *data)
>>  	struct i2c_client *client = data->client;
>>
>>  	sysfs_remove_group(&client->dev.kobj, &mxt_attr_group);
>> +	sysfs_remove_group(&client->dev.kobj, &mxt_fw_attr_group);
>>  }
>>
>>  static void mxt_reset_slots(struct mxt_data *data)
>> @@ -4649,31 +4650,19 @@ static int mxt_remove(struct i2c_client *client)
>>  {
>>  	struct mxt_data *data = i2c_get_clientdata(client);
>>
>> -	mutex_lock(&data->lock);
>> -	if (data->e_state == MXT_STATE_UPDATING_CONFIG_ASYNC ||
>> -	    data->e_state == MXT_STATE_UPDATING_CONFIG) {
>> -		data->e_state = MXT_STATE_GOING_AWAY;
>> -		mutex_unlock(&data->lock);
>> -		mxt_wait_for_completion(data, &data->update_cfg_completion,
>> -					MXT_CONFIG_TIMEOUT);
>> -	} else {
>> -		data->e_state = MXT_STATE_GOING_AWAY;
>> -		mutex_unlock(&data->lock);
>> -	}
>> +	mxt_sysfs_remove(data);
>>
>> -	disable_irq(data->irq);
>> -	sysfs_remove_group(&client->dev.kobj, &mxt_fw_attr_group);
>>  	if (data->reset_gpio) {
>>  		sysfs_remove_link(&client->dev.kobj, "reset");
>>  		gpiod_unexport(data->reset_gpio);
>>  	}
>> +
>>  	mxt_debug_msg_remove(data);
>> -	mxt_sysfs_remove(data);
>>  	mxt_free_input_device(data);
>>  	mxt_free_object_table(data);
>>
>> 	if (debug_state)
>> 		cancel_delayed_work_sync(&data->watchdog_work);
>> +	disable_irq(data->irq);
>>
>>  	return 0;
>>  }
>>
> 
> I'm looking at this again and the original tear-down order of the
> mxt_remove() looks okay, so no need to change it.
> 
> Reading the commit message, it says that request_firmware_nowait() races
> with kfree(data), but that can't happen because the data is
> resource-managed and request_firmware_nowait() bumps device's use-count.
> 
> https://elixir.bootlin.com/linux/v5.6.2/source/drivers/base/firmware_loader/main.c#L1043
> 
> I think this patch was ported from some very old kernel version and it's
> simply not applicable to the upstream anymore.
> 

Moreover, request_firmware_nowait() uses try_module_get(), hence
driver's module can't be unloaded until firmware is loaded. This patch
is wrong, please drop it.