App hangs in QPixmap::fromImage

mpouchol

Hi,

I'm using a Qt app to display video stream from a grayscale devicealab camera. Images are displayed in a QGraphicsView using QPixmap. For some reason app randomly gets stuck in QPixmap::fromImage!

Environment:
Qt 5.15.0 msvc2019 x64

Execution flow:

acquire images in camera thread
in UI thread, at refresh rate make a copy of image buffer, then create a QImage from buffer data
create QPixmap from QImage
set QPixmap to a QGraphicsPixmapItem

Call stack:
Using Qt's dlls with pdb I see that Qt waits for a semaphore to be released after image buffer conversion:

Image conversion:
From what I see in Qt codebase, the convert_generic function uses a thread pool to perform image buffer conversion. One of the threads doesn't release the semaphore so UI hangs.

Any help greatly appreciated.
Thanks

JonB

@mpouchol said in App hangs in QPixmap::fromImage:

in UI thread, at refresh rate make a copy of image buffer, then create a QImage from buffer data

Hello and welcome.

May not be the issue/relevant, but just checking: how do you "make a copy of image buffer" safely, given that camera thread is updating image buffer? Is that code thread-safe?

mpouchol

Hello @JonB ,

This copy is protected with a mutex.
The camera thread itself keeps on getting images from the camera, only the main thread is frozen.

Christian Ehrlicher

Look in your other threads where you also try to convert a QImage and hang somewhere near convert_generic()

mpouchol

@Christian-Ehrlicher the QImage to QPixmap conversion is only done in the main/UI thread.

Christian Ehrlicher

@mpouchol said in App hangs in QPixmap::fromImage:

the QImage to QPixmap conversion is only done in the main/UI thread.

This does not what I've suggested... go through your other threads and take a look what they're doing and if there's another one in this function.

mpouchol

@Christian-Ehrlicher thanks for your suggestion. I'm not sure to understand what I should do.
The convert_generic function is actually one of the function called by QPixmap::fromImage as you can see in the callstack attached to my question.
The app is stuck in this function trying to acquire a semaphore.

JonB

@mpouchol
@Christian-Ehrlicher is asking you to grep all your code for every occurrence of a call to fromImage() and/or convertToFormat() to see whether you call it in more than one place.

mpouchol

Thanks @JonB, I only use QPixmap::fromImage in the main thread. I do not call convertToFormat() directly, it's called in fromImage.

mchinand

Is your QImage good? Does saving the QImage to a file work? What is the QImage::Format of the image?

mpouchol

I not sure about the QImage, I will try to save it. It's mostly a temporary wrapper around the buffer data that's used to create a QPixmap. The format is QImage::Format_Grayscale8

mchinand

Also, does it sometimes work and sometimes hang or always hang? Are you possibly not getting a full frame of data from the camera? Each frame of data should be the same size (number of bytes), right?

mpouchol

It works most of the time and hangs randomly. It can take somewhere between a few seconds and a 1-2 hours to hang. Images are fetched from camera at around 60fps and each frame should be the same size. I suspect data gets corrupted somewhere but I was expecting a crash instead of an app freeze. It's a bit strange that it always hang at the same point.

Christian Ehrlicher

Again: it hangs on a mutex lock - nothing to do with the data. Take a look if this mutex is locked in another thread...

mpouchol

@Christian-Ehrlicher the mutex is owned by Qt code inside QPixmap::fromImage
I don't use this function from other threads so I don't see what I did that caused it to stay locked.

Christian Ehrlicher

@mpouchol said in App hangs in QPixmap::fromImage:

I don't use this function from other threads so I don't see what I did that caused it to stay locked.

I'm giving up... looks like it's too hard to look into the other threads while it is hanging to see if another thread is also using this mutex. So try to figure it out by your own.

mpouchol

@Christian-Ehrlicher said in App hangs in QPixmap::fromImage:

I'm giving up... looks like it's too hard to look into the other threads

I did that and that's not the case. Thanks for your help anyway.

Christian Ehrlicher

Ok, taking a deeper look into convert_generic() the qimage is converted with more than one thread. One of these subthreads hangs maybe due to a broken QImage. Look into your other threads to see which one is somewhere hanging inside convertSegment()

mpouchol

I also had a look at the code inside convert_generic(). When the main thread is frozen I don't see other threads hanging in convertSegment which make me think that maybe one of the convertSegment threads has died silently and is not releasing the semaphore in convert_generic because I don't see visual studio raising exceptions in other threads.

Christian Ehrlicher

If you compile Qt by your own build a big try/catch block around convert_segment and simply catch all exceptions to see if there is one.
You can also create a helper thread which writes out the current qimage after a certain period of time when it was not canceled in between so you can inspect the qimage.