signals, syscalls and schedule()

[zerons]

Hi all.
I have several doubts about signals and syscalls.

When a signal handler gets involved?
About signal, I think I have figured it out, maybe?
I checked the source code, and found that when call `prepare_exit_to_usermode`
-> `exit_to_usermode_loop`, then `do_signal` gets involved. So when switch to
user mode, the signal handler get called before restart the syscall if needed.
That means when return from a syscall or the process scheduled, we could handle
the signal.

Interruptible syscalls, they check the process signal state by themselves and
return -ERESTARTSYS or -EINTR, is that why we call them interruptible syscalls?

When a syscall running, and then the time slice runs out, does it switch to a
new process or wait until the system call returns? If switch to a new process,
when the interrupted process runs again, is it still in kernel mode to finish
the syscall? Is that to say, a signal is handled when return from a syscall or
the process scheduled and it is in usermode?