The Linux Signals Handling Model
Signals can be sent from system calls, interrupts and bottom-half handlers (see sidebar) alike; there is no difference. In other words, the Linux signal queue is interrupt-safe, as strange and recursive as that sounds, so it's fairly flexible.
An interesting signal-delivery case, however, is on SMP. Imagine a thread is executing on one processor, and it gets an asynchronous event (e.g., synchronous socket I/O signal) from an IRQ handler (or another process) on another CPU. In that case, we send a cross-CPU message to the running process, so there is no latency in signal delivery. (The speed of cross-CPU delivery is about five microseconds on a Pentium II 350MHz.)
Once again, we notice how Linux is actually the technology leader in important kernel aspects such as scheduling, interrupt handling and signals handling. This also proves the conjecture that the Linux developer community is collectively more capable and more resourceful than any private corporation's R&D department could ever be.
Moshe Bar (firstname.lastname@example.org) is an Israeli system administrator and OS researcher, who started learning UNIX on a PDP-11 with AT&T UNIX Release 6 back in 1981. He holds an M.Sc. in computer science. Visit Moshe's web site at http://www.moelabs.com/.
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