Re: [PATCH 1/2] HID: logitech-hidpp: Avoid hidpp_connect_event() running while probe() restarts IO

On Oct 06 2023, Hans de Goede wrote:

Hi Benjamin,

On 10/6/23 15:46, Benjamin Tissoires wrote:

quoted

Hi Hans,

On Oct 06 2023, Hans de Goede wrote:

quoted

hidpp_probe() restarts IO after setting things up, if we get a connect
event just before hidpp_probe() stops all IO then hidpp_connect_event()
will see IO errors causing it to fail to setup the connected device.

I think I see why you are doing this, but it scares me to be honest.

quoted

Add a new io_mutex which hidpp_probe() locks while restarting IO and
which is also taken by hidpp_connect_event() to avoid these 2 things
from racing.

So now we are adding a new mutex to prevent a workqueue to be executed,
which is held while there is another semaphore going down/up...
It feels error prone to say the least and I'm not sure we are not
actually fixing the problem.

My guts tells me that the issue is tackled at the wrong time, and that
maybe there is a better solution that doesn't involve a new lock in the
middle of 2 other locks being actually held...

Since the lock is only taken into 2 places and 1 of them is holding
no locks when taking it (because workqueue) I don't really see how
this would be a problem.

Actually introducing a new lock for this, rather then say trying
to use the send_mutex makes this much easier to reason about,
more on this below.

quoted

One minor comment in the code.

quoted

Hopefully this will help with the occasional connect errors leading to
e.g. missing battery monitoring.

Cc: <redacted>
Signed-off-by: Hans de Goede <redacted>
---
 drivers/hid/hid-logitech-hidpp.c | 35 ++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
index a209d51bd247..33f9cd98485a 100644
--- a/drivers/hid/hid-logitech-hidpp.c
+++ b/drivers/hid/hid-logitech-hidpp.c

@@ -181,6 +181,7 @@ struct hidpp_scroll_counter {
 struct hidpp_device {
 	struct hid_device *hid_dev;
 	struct input_dev *input;
+	struct mutex io_mutex;
 	struct mutex send_mutex;
 	void *send_receive_buf;
 	char *name;		/* will never be NULL and should not be freed */

@@ -4207,36 +4208,39 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
 		return;
 	}
 
+	/* Avoid probe() restarting IO */
+	mutex_lock(&hidpp->io_mutex);

I'd put a `__must_hold(&hidpp->io_mutex);` here, not changing any return
path and forcing any caller to `hidpp_connect_event()` (which will
eventually only be the work struct) to take the lock.

This should simplify the patch by a lot and also ensure someone doesn't
forget the `goto out_unlock`.

Ok, I can add the __must_hold() here and make 
delayed_Work_cb take the lock, but that would make it
impossible to implement patch 2/2 in a clean manner and
I do like patch 2/2 since it makes it clear that
hidpp_connect_event must only run from the workqueue
but I guess we could just add a comment for that
instead.

In 2/2, just rename this function to __do_hidpp_connect_event(), and
have hidpp_connect_event() being the worker, which takes the lock, and
calls __do_hidpp_connect_event().

Either way works for me, with a slight preference
for the current version even if it introduces
a bunch of gotos.

quoted

quoted

+
 	if (hidpp->quirks & HIDPP_QUIRK_CLASS_WTP) {
 		ret = wtp_connect(hdev, connected);
 		if (ret)
-			return;
+			goto out_unlock;
 	} else if (hidpp->quirks & HIDPP_QUIRK_CLASS_M560) {
 		ret = m560_send_config_command(hdev, connected);
 		if (ret)
-			return;
+			goto out_unlock;
 	} else if (hidpp->quirks & HIDPP_QUIRK_CLASS_K400) {
 		ret = k400_connect(hdev, connected);
 		if (ret)
-			return;
+			goto out_unlock;
 	}
 
 	if (hidpp->quirks & HIDPP_QUIRK_HIDPP_WHEELS) {
 		ret = hidpp10_wheel_connect(hidpp);
 		if (ret)
-			return;
+			goto out_unlock;
 	}
 
 	if (hidpp->quirks & HIDPP_QUIRK_HIDPP_EXTRA_MOUSE_BTNS) {
 		ret = hidpp10_extra_mouse_buttons_connect(hidpp);
 		if (ret)
-			return;
+			goto out_unlock;
 	}
 
 	if (hidpp->quirks & HIDPP_QUIRK_HIDPP_CONSUMER_VENDOR_KEYS) {
 		ret = hidpp10_consumer_keys_connect(hidpp);
 		if (ret)
-			return;
+			goto out_unlock;
 	}
 
 	/* the device is already connected, we can ask for its name and

@@ -4245,7 +4249,7 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
 		ret = hidpp_root_get_protocol_version(hidpp);
 		if (ret) {
 			hid_err(hdev, "Can not get the protocol version.\n");
-			return;
+			goto out_unlock;
 		}
 	}
 

@@ -4256,7 +4260,7 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
 						   "%s", name);
 			kfree(name);
 			if (!devm_name)
-				return;
+				goto out_unlock;
 
 			hidpp->name = devm_name;
 		}

@@ -4291,12 +4295,12 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
 
 	if (!(hidpp->quirks & HIDPP_QUIRK_DELAYED_INIT) || hidpp->delayed_input)
 		/* if the input nodes are already created, we can stop now */
-		return;
+		goto out_unlock;
 
 	input = hidpp_allocate_input(hdev);
 	if (!input) {
 		hid_err(hdev, "cannot allocate new input device: %d\n", ret);
-		return;
+		goto out_unlock;
 	}
 
 	hidpp_populate_input(hidpp, input);

@@ -4304,10 +4308,12 @@ static void hidpp_connect_event(struct hidpp_device *hidpp)
 	ret = input_register_device(input);
 	if (ret) {
 		input_free_device(input);
-		return;
+		goto out_unlock;
 	}
 
 	hidpp->delayed_input = input;
+out_unlock:
+	mutex_unlock(&hidpp->io_mutex);
 }
 
 static DEVICE_ATTR(builtin_power_supply, 0000, NULL, NULL);

@@ -4450,6 +4456,7 @@ static int hidpp_probe(struct hid_device *hdev, const struct hid_device_id *id)
 		will_restart = true;
 
 	INIT_WORK(&hidpp->work, delayed_work_cb);
+	mutex_init(&hidpp->io_mutex);
 	mutex_init(&hidpp->send_mutex);
 	init_waitqueue_head(&hidpp->wait);
 

@@ -4519,6 +4526,9 @@ static int hidpp_probe(struct hid_device *hdev, const struct hid_device_id *id)
 	flush_work(&hidpp->work);
 
 	if (will_restart) {
+		/* Avoid hidpp_connect_event() running while restarting */
+		mutex_lock(&hidpp->io_mutex);
+
 		/* Reset the HID node state */
 		hid_device_io_stop(hdev);

That's the part that makes me raise an eyebrow. Because we lock, then
release the semaphore to get it back later. Can this induce a dead lock?

Can't we solve that same scenario without a mutex, but forcing either
the workqueue to not run or to be finished at this point?

I'm not sure what you are worried about after the mutex_lock
the line above we are 100% guaranteed that hidpp_connect_event()
is not running and since it is not running it will also not
be holding any other locks, so it can not cause any problems.

Agree, but my point is that you are not entirely solving the issue:
if now, between hid_device_io_stop() and hid_hw_close() we receive a
connect notification from the device, hid_input_report() will return
-EBUSY, and we will lose it (it will not be stacked in the workqueue).

I was thinking at adding a flush_work(&hidpp->work) here, instead of
the mutex solution, but yours ensures that any connect event already
started will be handled properly, which is a plus.

Still if between the mutex lock here we receive a connect event from the
device, we still get -EBUSY at the hid-core layer, and so we will lose
it. Maybe that's OK because we might re-ask for the device later (I
don't remember exactly the code), but my point is that because we add a
mutex doesn't mean we will solve all multi-thread problems. So finding a
non-mutex solution sometimes is better :)

And the fact that we need to think through every preemption case often
means that there is something wrong *elsewhere*.

The other way around if hidpp_connect_event() is running
the mutex_lock() above ensures that it will finishes and
release any locks before we get here.

There is no way that both code paths can run at the same time
with the new lock. And there thus also is no way that they
can cause any new not already held locks to be taken while
the other side is running.

I actually introduced the new lock because in my mind
introducing the new lock allows to easily reason about
the impact on other locking (which is none).

I hope this helps explain. As for the making
hidpp_connect_event()'s caller take the lock thing, let me
know how you want to resolve that. Either way works for me
and I guess the less intrusive version of making the caller
take the lock is easier to backport so we should probably
go that route.

Again, for that particular point, making the function a private one that
doesn't have to cope with the mutex will be simpler in this patch and in
the future.

Cheers,
Benjamin

Regards,

Hans

quoted

quoted

 		hid_hw_close(hdev);

@@ -4529,6 +4539,7 @@ static int hidpp_probe(struct hid_device *hdev, const struct hid_device_id *id)
 
 		/* Now export the actual inputs and hidraw nodes to the world */
 		ret = hid_hw_start(hdev, connect_mask);
+		mutex_unlock(&hidpp->io_mutex);
 		if (ret) {
 			hid_err(hdev, "%s:hid_hw_start returned error\n", __func__);
 			goto hid_hw_start_fail;

@@ -4553,6 +4564,7 @@ static int hidpp_probe(struct hid_device *hdev, const struct hid_device_id *id)
 	sysfs_remove_group(&hdev->dev.kobj, &ps_attribute_group);
 	cancel_work_sync(&hidpp->work);
 	mutex_destroy(&hidpp->send_mutex);
+	mutex_destroy(&hidpp->io_mutex);
 	return ret;
 }
 

@@ -4568,6 +4580,7 @@ static void hidpp_remove(struct hid_device *hdev)
 	hid_hw_stop(hdev);
 	cancel_work_sync(&hidpp->work);
 	mutex_destroy(&hidpp->send_mutex);
+	mutex_destroy(&hidpp->io_mutex);
 }
 
 #define LDJ_DEVICE(product) \

-- 
2.41.0

Cheers,
Benjamin