Linux – Can’t kill user process

I'm sorry, but this simply is not possible (that's by design). However, if members of a common group, user1 could write to a file that user2's process checks, indicating to the process that it should terminate.

Or, user2 could run something in the background that checks a file, then sends the appropriate signals. User1 then simply has to write to that file. This may be easier, as it would not require any modification of user2's programs.

Conventionally, no, user1 can not send POSIX signals to user2's process.

A process in S or D state is usually in a blocking system call, such as reading or writing to a file or the network, waiting for a called program to finish, or while waiting on semaphores or other synchronization primitives. It will go into the sleep state while waiting.

You can't "wake it up" - it will only proceed when the data/resource it is waiting for becomes available. This is all normal and expected, and only a problem when trying to kill it.

You can try and use strace -p pid to find out which system call is currently happening for process pid.

From wikipedia :

An uninterruptible sleep state is a sleep state that won't handle a signal right away. It will wake only as a result of a waited-upon resource becoming available or after a time-out occurs during that wait (if specified when put to sleep). It is mostly used by device drivers waiting for disk or network IO (input/output). When the process is sleeping uninterruptibly, signals accumulated during the sleep will be noticed when the process returns from the system call or trap.

A process blocked in a system call is in uninterruptible sleep, which as its name says, is really uninterruptible even by root.

Normally, processes cannot block SIGKILL. But kernel code can, and processes execute kernel code when they call system calls, during which Kernel code blocks all signals. So if a system call blocks indefinitely, there may effectively be no way to kill the process. The SIGKILL will only take effect whenever the process completes the system call.