[BUG] vsock: Mem leak caused by circular referencing

[cutelizebin]

There might be a memory leak bug caused by circular referencing.

VhostUserBackend ==> VhostUserVsockThread ==> VringEpollHandler
^ V
|| ||
==================================

There three struct refers to each other using Arc , but no single line is using Weak, which make it a circle.

And all these three struct cannot be dropped properly.

Reproduction Process:

1. Launch vhost-device-vsock
2. Launch and kill QEMU or other VMM
3. Use top -Hp $(pgrep vhost-device-vsock) to watch the threads details

Advice

Maybe we can use Weak<VhostUserBackend> in VringEpollHandler.

[stefano-garzarella]

Maybe we can use Weak<VhostUserBackend> in VringEpollHandler.

Or what about in VhostUserVsockThread where we're already using Option<Arc<VringEpollHandler<ArcVhostBknd, VringRwLock, ()>> ?

@cutelizebin do you want to work on it?

[cutelizebin]

yes，I'll send a fix later on.

[stefano-garzarella]

This is also causing an issue after we merged commit 38caab2

When the VM reboot or shutdown, we have this error, instead of looping waiting for a new VM

ERROR vhost_device_vsock] Could not create backend: CID already in use by another vsock device

This happens because we have these cyclic references and VhostUserVsockThread::drop() is never invoked. So we don't remove the cid from the map.

[stefano-garzarella]

@cutelizebin do you have time to work on it? Otherwise, I can take a look ;-)

[stefano-garzarella]

@cutelizebin was easier than expected, we don't need to keep the VringEpollHandler reference in VhostUserVsockThread. I just sent #499 .

Thanks for reporting this!