Question

I have a test case where there are threads spawned using CLONE_THREAD option in clone() .Here if i want to kill a particular thread I suppose we should be using SYS_tgkill in systemcall(). But will the kill actually affect a thread if it is waiting in kernel space(say a futex_wait)?

I tried killing a thread created in the above manner.But when SIGKILL is sent to the same the whole process is getting killed.Am i missing something in using syscall(SYS_tgkill,pid,tid,9) ?

Answer 1

SIGKILL always kills the target process. There is no way around this; it’s unblockable, unignorable, and uncatchable.

You could try sending another signal (like SIGUSR1 or SIGHUP or SIGRTMIN) and having a signal handler installed that calls pthread_exit (but note that this function is not async-signal-safe, so you must ensure that the signal handler did not interrupt another async-signal-unsafe function) or use cancellation (pthread_cancel) to stop the blocked thread.

This should work for normal blocking operations (like waiting for data from a pipe or socket), but it will not help you if the thread is in an uninterruptable sleep state (like trying to read from a badly scratched CD or failing hard disk).