Cargo Package Cache is Locked?

jlb6740@jlb6740:~/my-sys$ cargo check
Blocking waiting for file lock on package cache

Not sure this is the best place for Cargo question but could not find my answer searching with google.
What package cache is cargo referring to (I assume it some location where packages in cargo.toml are downloaded to)?
How do I find out what is holding the lock and how do I release that lock?

BTW .. I have gotten rid of the error before by just doing a git reset --hard, but it has come back where doing a git reset --hard on my project doesn't work. I would like to understand this message.

Thanks.

Ok. Found a cache file here ~/.cargo/.package-cache that was empty. I deleted it and the build works again. I am not sure the steps that got me to this error but I assume deleting this file will fix it everytime.