What version of Go are you using (go version)?

$ go version
go version go1.11.5 linux/amd64

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

$ go env
GOHOSTARCH="amd64"
GOHOSTOS="linux"

What did you do?

I am wondering whether the following steps are safe:

1. in one go routine lock Mutex M, and then pass M's pointer and a golang exported function(with //export functionName above the function) to a cgo function, lock M again. The go routine will be blocked.
2. after the c library recived the M's pointer and exported function, it starts to do some background job, after the job is completed, it will call the exported function and pass M's pointer as a parameter.
3. the exported function will unlock the Mutex
4. the blocked go routine will be unblocked.

You could find the specific code in this patch, if I do not make it clear.

The sync.Mutex is locked in one go routine and uncloked is called in a go exported function which is called by a c library. Will the exported function be scheduled to the original OS thread which owns the blocked go routine? Or is lock/unlock sync.Mutex in different OS thread safe? I know it's safe to lock/unlock sync.Mutex in different go routines, but lock/unlock mutex(OS) in different pthreads is not safe.