sys_tgkill - send signal to one specific thread
long sys_tgkill (int tgid, int pid, int sig);
tgid the thread group ID of the thread pid the PID of the thread sig signal to be sent
This syscall also checks the tgid and returns -ESRCH even if the PID exists but it’s not belonging to the target process anymore. This method solves the problem of threads exiting and PIDs getting reused.