linux-kernel - Re: [PATCH v10 54/55] Input: atmel_mxt_ts: Implement synchronization during various operation

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Date:   Thu, 2 Apr 2020 16:24:38 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     "Wang, Jiada" <jiada_wang@...tor.com>, nick@...anahar.org,
        dmitry.torokhov@...il.com, jikos@...nel.org,
        benjamin.tissoires@...hat.com, bsz@...ihalf.com
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        erosca@...adit-jv.com, Andrew_Gabbasov@...tor.com
Subject: Re: [PATCH v10 54/55] Input: atmel_mxt_ts: Implement synchronization
 during various operation

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@...tor.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;
 }