Processes 4: Interruption and race conditions

Overview

In this lecture, we discuss the Unix analogue of kernel interrupts, namely signals, and race conditions. This is a bridge lecture to the synchronization unit.

Full lecture notes — Textbook readings

The simple life

• Your program has exactly one task to perform
• If a subtask blocks, the program can wait for it indefinitely
• If a subtask aborts or receives inappropriate input, the program can abort
• Exits when done
• Examples: cat, sleep, wc, tr, echo…

Multitasking

• Your program has multiple tasks to perform
• Manage parallel subtasks
• If a subtask blocks or aborts, the program may need to cancel that subtask or otherwise respond
• The program must handle inappropriate input
• May not exit when done
• Examples: sh, Web servers…

• But tools and techniques will help!

Race conditions

• A race condition is a bug that can manifest depending on timing
  • (More generally, it’s any unexpected system behavior dependent on scheduling, but the term generally refers to bugs)

Example: Relay race

• Run echo and cat in parallel
• cat should read what echo wrote

Relay race attempt 1

$ /bin/echo Handoff | cat

https://www.thestar.com/sports/olympics/2016/08/18/why-the-us-womens-4x100-relay-team-ran-by-themselves-on-the-track.html

Relay race attempt 2

$ /bin/echo Handoff > handoff.txt & cat handoff.txt

https://www.dailymail.co.uk/sport/olympics/article-2185586/London-Olympics-2012-Great-Britain-looking-overcome-baton-woes-relays.html

https://www.stabroeknews.com/2016/08/19/sports/u-s-grasp-second-chance-4x100-relay/

Explanation

• In /bin/echo Handoff > handoff.txt & cat handoff.txt, the two programs run in parallel
  • They attempt to synchronize using a file, handoff.txt
  • But handoff.txt can be read or written at any time
  • Maybe cat reads before echo writes!
• In /bin/echo Handoff | cat, the two programs run in parallel
  • They attempt to synchronize using a pipe
  • But reading from an empty pipe blocks the caller!
  • cat will always read what echo wrote

Reasoning about race conditions

• Which actions can occur in parallel?
• Which actions block other actions?
• Waits-for graph

Signals

• A signal is the process control analogue for an operating system interrupt
• Used to represent events that might occur at unpredictable times and/or need to interrupt long-running computations
  • Control-C ⟶ SIGINT
  • kill -9 ⟶ SIGKILL
  • Null pointer reference ⟶ SIGSEGV
  • A child process exited ⟶ SIGCHLD

Signal system calls

• sigaction establishes a signal handler

void handle_signal(int signal_number) {
    // do something to handle the signal
}

...
struct sigaction sa;
sa.sa_handler = handle_signal; // or SIG_IGN or SIG_DFL
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
sigaction(SIGINT, &sa, nullptr);

• kill(pid_t pid, int sig) sends a signal
  • Some signals are generated automatically

When can a signal be delivered?

• In between any two instructions in the program
• Also interrupts certain system calls
  • System call may return early (e.g., a short read)
  • System call may return having done no work: errno == EINTR
• man 7 signal for more
  • “Interruption of system calls…” for list of system calls that can return EINTR
  • Also documented on system call manual pages

Timed wait

• Parent process starts a child
• Waits for child to exit or 0.75 sec, whichever comes first
• Questions
  • Possible race conditions?
  • Solution?

timedwait-poll

• Reliable, but uses 100% CPU to wait
  • Not a valuable use of CPU

Timed wait arguments

• -e TIME: child exits after TIME (default 0.5 sec)
• -t TIME: parent timeout is TIME (default 0.75 sec)
• -q: quiet output

Polling and blocking

• Blocking: Process waits for communication
• Polling: Process checks repeatedly for communication
• Advantage of polling: Fewer race conditions
• Advantage of blocking: Lower CPU usage

timedwait-block

• Unreliable!
• If child exits immediately, signal is delivered before usleep, and therefore does not interrupt usleep

timedwait-blockvar

• Still unreliable! (though less unreliable)
• Signal might delivered between any two instructions!

Race conditions!

• Can you use synchronous IPC to solve this race condition?

timedwait-selfpipe

• Process opens pipe to itself
• Signal handlers, for either SIGALRM (timeout) or SIGCHLD (child exit), write to pipe
• read will either succeed right away or be interrupted by a signal
  • Reliable timeout!