The ff_mutex_create function is for mutal exclusion control of file system object. It is called on register a file system object and create its mutex.
int ff_mutex_create ( int vol /* [IN] Volume ID */ );
A non-zero value is returned if a mutex has been created. If the mutex is not available, a zero is returned and f_mount function will fail with FR_INT_ERR.
This function is required when FF_FS_REENTRANT = 1.