Lecture 08: Race Conditions, Deadlock, and Data Integrity

Principles of Computer Systems

Spring 2019

Stanford University

Computer Science Department

Lecturer: Chris Gregg

PDF of this presentation

  • Comment on the end of last Wednesday's lecture:

Lecture 07 (review): Masking Signals and Deferring Handlers

// job-list-fixed.c
static void reapProcesses(int sig) {
  while (true) {
    pid_t pid = waitpid(-1, NULL, WNOHANG);
    if (pid <= 0) break;
    printf("Job %d removed from job list.\n", pid);
  }
}

char * const kArguments[] = {"date", NULL};
int main(int argc, char *argv[]) {
  signal(SIGCHLD, reapProcesses);
  sigset_t set;
  sigemptyset(&set);
  sigaddset(&set, SIGCHLD);
  for (size_t i = 0; i < 3; i++) {
    sigprocmask(SIG_BLOCK, &set, NULL);
    pid_t pid = fork();
    if (pid == 0) {
      sigprocmask(SIG_UNBLOCK, &set, NULL);
      execvp(kArguments[0], kArguments);
    }
    sleep(1); // force parent off CPU
    printf("Job %d added to job list.\n", pid);
    sigprocmask(SIG_UNBLOCK, &set, NULL);
  }
  return 0;
}
  • In discussing the job-list-fixed example (here), we discussed whether the child's signal handler could get called if the program that the child launched with execvp had a child of its own, and that child ended.
  • I mistakenly discussed what might need to be done to avoid this, but it turns out that nothing needs to be done!
  • Once the original child starts another program with execvp all of the original code is gone. Therefore, the signal handler cannot be called, because it doesn't exist any longer.
  • This is distinct from the idea that blocked signals are still blocked across an execvp boundary.
int kill(pid_t pid, int signum);
int raise(int signum); // equivalent to kill(getpid(), signum);
  •  Signal extras: kill and raise
    • Processes can message other processes using signals via the kill system call. And processes can even send themselves signals using raise.

       
    • The kill system call is analogous to the /bin/kill shell command.
      • Unfortunately named, since kill implies SIGKILL implies death.
      • So named, because the default action of most signals in early UNIX implementations was to just terminate the target process.
    • We generally ignore the return value of kill and raise. Just make sure you call it properly.
    • The pid parameter is overloaded to provide more flexible signaling.
      • When pid is a positive number, the target is the process with that pid.
      • When pid is a negative number less than -1, the targets are all processes within the process group abs(pid). We'll rely on this in Assignment 4.
      • pid can also be 0 or -1, but we don't need to worry about those. See the man page                  for kill if you're curious.

Lecture 07 (review): Masking Signals and Deferring Handlers

  •  The job-list-broken and job-list-fixed examples from the prior slide deck highlight a key issue that comes with the introduction of signals and signal handling.
    • Neither job-list-broken nor job-list-fixed can anticipate when a child process will finish up. That means it has no control over when SIGCHLD signals arrive.
    • Processes do, however, have some control over how they respond to SIGCHLD signals.
      • They install custom SIGCHLD handlers to surface information about what process exited. We've seen a lot of that already.
      • When a process elects to use signal handling, it shouldn't be penalized by having to live with the concurrency issue that come with it. That would only encourage programmers to avoid signals and signal handling, even when it's the best thing to do.
      • That's why the kernel provides the option to defer a signal handler to run only when it can't cause problems. That's what our job-list-fixed program does.
      • It's true that the program could abuse the power to block signals for longer than necessary, but we have no choice but to assume the program wants to use signal handlers properly, else they wouldn't be installing them in the first place.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

  •  Let's revisit the simplesh example from last week. The full program is right here.

 

 

 

 

 

 

 

 

  • The problem to be addressed: Background processes are left as zombies for the lifetime of the shell. At the time we implemented simplesh, we had no choice, because we hadn't learned about signals or signal handlers yet.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

// simplesh.c
int main(int argc, char *argv[]) {
  while (true) {
    // code to initialize command, argv, and isbg omitted for brevity                                                                                                                                                              
    pid_t pid = fork();
    if (pid == 0) execvp(argv[0], argv);
    if (isbg) {
      printf("%d %s\n", pid, command);
    } else {
      waitpid(pid, NULL, 0);                                                                                                                                                                        
    }
  }
  printf("\n");
  return 0;
}
  • Now we know about SIGCHLD signals and how to install SIGCHLD handlers to reap zombie processes.  Let's upgrade our simplesh implementation to reap all process resources.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

// simplesh-with-redundancy.c                                                                                                                                                           
static void reapProcesses(int sig) {
  while (waitpid(-1, NULL, WNOHANG) > 0) {;} // nonblocking, iterate until retval is -1 or 0
}

int main(int argc, char *argv[]) {
  signal(SIGCHLD, reapProcesses);
  while (true) {
    // code to initialize command, argv, and isbg omitted for brevity                                                                                                                                                              
    pid_t pid = fork();
    if (pid == 0) {
      execvp(argv[0], argv);
      printf("%s: Command not found\n", argv[0]);
      exit(0);
    }
    if (isbg) {
      printf("%d %s\n", pid, command);
    } else {
      waitpid(pid, NULL, 0);                                                                                                                                                                      
    }
  }
  printf("\n");
  return 0;
}
  • The last version actually works, but it relies on a sketchy call to waitpid to halt the shell until its foreground process has exited.
    • When the user creates a foreground process, normal execution flow advances to an isolated waitpid call to block until that process has terminated.
    • When the foreground process finishes, however, the SIGCHLD handler is invoked, and its waitpid call is the one that culls the foreground process's resources.
    • When the SIGCHLD handler exits, normal execution resumes, and the original call to waitpid returns -1 to state that there is no trace of a process with the supplied pid.
    • The version on the last slide deck is effectively calling waitpid from main just to block until the foreground process vanishes.
    • Even if you're content with this unorthodox use of waitpid—i.e. invoking a system call when you know it will fail—the waitpid call is redundant and replicates functionality better managed in the SIGCHLD handler.
      • We should only be calling waitpid in one place: the SIGCHLD handler.
      • This will be all the more apparent when we implement shells (e.g. Assignment 4's stsh) where multiple processes are running in the foreground as part of a pipeline (e.g.                                               more words.txt | tee copy.txt | sort | uniq)

Lecture 08: Race Conditions, Deadlock, and Data Integrity

  • Here's an updated version that's careful to call waitpid from only one place.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

// simplesh-with-race-and-spin.c                                                                                                                                                           
static pid_t fgpid = 0; // global, intially 0, and 0 means no foreground process                                                                                                                                                            
static void reapProcesses(int sig) {
  while (true) {
    pid_t pid = waitpid(-1, NULL, WNOHANG);
    if (pid <= 0) break;
    if (pid == fgpid) fgpid = 0; // clear foreground process
  }
}

static void waitForForegroundProcess(pid_t pid) {
  fgpid = pid;
  while (fgpid == pid) {;}
}

int main(int argc, char *argv[]) {
  signal(SIGCHLD, reapProcesses);
  while (true) {
    // code to initialize command, argv, and isbg omitted for brevity                                                                                                                                                              
    pid_t pid = fork();
    if (pid == 0) execvp(argv[0], argv);
    if (isbg) {
      printf("%d %s\n", pid, command);
    } else {
      waitForForegroundProcess(pid);                                                                                                                                                                      
    }
  }
  printf("\n");
  return 0;
}
  • The version on the last page introduces a global variable called fgpid to hold the process is of the foreground process. When there's no foreground process, fgpid is 0.
    • Because we don't control the signature of reapProcesses, we have to choice but to make fgpid a global.
    • Every time a new foreground process is created, fgpid is set to hold that process's pid. The shell then blocks by spinning in place until fgpid is cleared by reapProcesses.
  • This version consolidates the waitpid code to reside in the handler and nowhere else.
  • This version introduces two serious problems, so it's far from an A+ solution.
    • It's possible the foreground process finishes and reapProcesses is invoked on its behalf before normal execution flow updates fgpid. If that happens, the shell will spin forever and never advance up to the shell prompt. This is a race condition, and race conditions are no-nos.
    • The while (fgpid == pid) {;} is also a no-no. This allows the shell to spin on the CPU even when it can't do any meaningful work.
      • It would be substantially better for simplesh to yield the CPU and to only be considered for CPU time when there's a chance the foreground process has exited.

         

Lecture 08: Race Conditions, Deadlock, and Data Integrity

  • The race condition can be cured by blocking SIGCHLD before forking, and only lifting that block after the global fgpid has been set.
    • Here's a version of the code that employs signal blocking to remove this race condition.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

// simplesh-with-spin.c                                                                                                                                                           
// code for reapProcesses omitted, because it's the same as before

static void waitForForegroundProcess(pid_t pid) {
  fgpid = pid;
  unblockSIGCHLD(); // lift only after fgpid has been set
  while (fgpid == pid) {;}
}

int main(int argc, char *argv[]) {
  signal(SIGCHLD, reapProcesses);
  while (true) {
    // code to initialize command, argv, and isbg omitted for brevity                                                                                                                                                              
    blockSIGCHLD();
    pid_t pid = fork();
    if (pid == 0) { 
      unblockSIGCHLD(); 
      execvp(argv[0], argv); 
    }
    if (isbg) {
      printf("%d %s\n", pid, command);
      unblockSIGCHLD();
    } else {
      waitForForegroundProcess(pid);                                                                                                                                                                      
    }
  }
}

// simples-utils.c
// includes a collection of helper functions

static void toggleSIGCHLDBlock(int how) {
  sigset_t mask;
  sigemptyset(&mask);
  sigaddset(&mask, SIGCHLD);
  sigprocmask(how, &mask, NULL);
}

void blockSIGCHLD() {
  toggleSIGCHLDBlock(SIG_BLOCK);
}

void unblockSIGCHLD() {
  toggleSIGCHLDBlock(SIG_UNBLOCK);
}

Note that we call unblockSIGCHLD in the child, before the execvp call.  We do so, because the child will otherwise inherit the signal block.

  • Race condition is now gone!
    • Note that we call blockSIGCHLD before fork, and we don't lift the block until fgpid has been set to the pid of the new foreground process.
    • We also call unblockSIGCHLD in the child right before the execvp call.
      • The child executable could very well depend on multiprocessing. If so, it would certainly call fork and rely on SIGCHLD signals and signal handling.
      • If we forget to call unblockSIGCHLD, the child process inherits the SIGCHLD block across the execvp boundary. That would compromise the child ability to work properly.
    • We also need to call unblockSIGCHLD for background processes. We do so after bookkeeping information is printf-ed to the screen, as we did for job-list-fixed.
    • We have not addressed the CPU spin issue, and we really need to.
      • We could change the while loop from while (fgpid == pid) {;}
        to while (fgpid == pid) {usleep(100000);}, as we have in this version.
      • usleep call will push the shell off the CPU every time it realizes it shouldn't have gotten it in the first place. But we'd really prefer to keep the shell off the CPU until the OS has some          information suggesting the foreground process is done.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

  • The C libraries provide a pause function, which forces the process to sleep until some unblocked signal arrives. This sounds promising, because we know fgpid can only be changed because a SIGCHLD signal comes in and reapProcesses is executed.
    • A version of simplesh whose waitForForegroundProcess implementation relies on pause is presented below on the left.
    • The problem here? SIGCHLD may arrive after fgpid == pid evaluates to true but before the call to pause it's committed to. That would be unfortunate, because it's possible simplesh isn't managing any other processes, which means that no other signals, much less SIGCHLD signals, will arrive to lift simplesh out of its pause call. That would leave simplesh in a state of deadlock.
    • You might think the second (lower right) version might help, but it has the same problem!

Lecture 08: Race Conditions, Deadlock, and Data Integrity

// simplesh-with-pause-1.c                                                                                                                                                           
static void waitForForegroundProcess(pid_t pid) {
  fgpid = pid;
  unblockSIGCHLD();
  while (fgpid == pid) {
    pause();
  }
}
// simplesh-with-pause-2.c                                                                                                                                                           
static void waitForForegroundProcess(pid_t pid) {
  fgpid = pid;
  while (fgpid == pid) {
    unblockSIGCHLD();
    pause();
    blockSIGCHLD();
  }
  unblockSIGCHLD();
}
  • The problem with both versions of waitForForegroundProcess on the prior slide is that each lifts the block on SIGCHLD before going to sleep via pause.
  • The one SIGCHLD you're relying on to notify the parent that the child has finished could very well arrive in the narrow space between lift and sleep. That would inspire deadlock.
  • The solution is to rely on a more specialized version of pause called sigsuspend, which asks that the OS change the blocked set to the one provided, but only after the caller has been forced off the CPU. When some unblocked signal arrives, the process gets the CPU, the signal is handled, the original blocked set is restored, and sigsuspend returns.






     
  • This is the model solution to our problem, and one you should emulate in your Assignment 3 farm    and your Assignment 4 stsh.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

// simplesh-all-better.c                                                                                                                                                           
static void waitForForegroundProcess(pid_t pid) {
  fgpid = pid;
  sigset_t empty;
  sigemptyset(&empty);
  while (fgpid == pid) {
    sigsuspend(&empty);
  }
  unblockSIGCHLD();
}
  • Let's go through an example that is the kind of signals problem you may see on the midterm exam.
  • Indeed, the problem is from a past midterm in CS 110:
    • Consider this program and its execution. Assume that all processes run to completion, all system and printf calls succeed, and that all calls to printf are atomic. Assume nothing about scheduling or time slice durations.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

static void bat(int unused) { 
  printf("pirate\n"); 
  exit(0);
}

int main(int argc, char *argv[]) {
  signal(SIGUSR1, bat);
  pid_t pid = fork();
  if (pid == 0) { 
    printf("ghost\n"); 
    return 0;
  }
  kill(pid, SIGUSR1); 
  printf("ninja\n"); return 0;
}
  • For each of the five columns, write a yes or no in the header line. Place a yes if the text below it represents a possible output, and place a no otherwise.
  • Let's go through an example that is the kind of signals problem you may see on the midterm exam.
  • Indeed, the problem is from a past midterm in CS 110:
    • Consider this program and its execution. Assume that all processes run to completion, all system and printf calls succeed, and that all calls to printf are atomic. Assume nothing about scheduling or time slice durations.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

static void bat(int unused) { 
  printf("pirate\n"); 
  exit(0);
}

int main(int argc, char *argv[]) {
  signal(SIGUSR1, bat);
  pid_t pid = fork();
  if (pid == 0) { 
    printf("ghost\n"); 
    return 0;
  }
  kill(pid, SIGUSR1); 
  printf("ninja\n"); return 0;
}
  • For each of the five columns, write a yes or no in the header line. Place a yes if the text below it represents a possible output, and place a no otherwise.
  • Let's go through another example that is the kind of signals problem you may see on the midterm exam.
    • Consider this program and its execution. Assume that all processes run to completion, all system and printf calls succeed, and that all calls to printf are atomic. Assume nothing about scheduling or time slice durations.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

int main(int argc, char *argv[]) {
    pid_t pid;
    int counter = 0;
    while (counter < 2) {
        pid = fork();
        if (pid > 0) break;
        counter++;
        printf("%d", counter);
    }
    if (counter > 0) printf("%d", counter);
    if (pid > 0) {
        waitpid(pid, NULL, 0);
        counter += 5;
        printf("%d", counter);
    }
    return 0;
}
  • List all possible outputs
  • Let's go through another example that is the kind of signals problem you may see on the midterm exam.
    • Consider this program and its execution. Assume that all processes run to completion, all system and printf calls succeed, and that all calls to printf are atomic. Assume nothing about scheduling or time slice durations.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

int main(int argc, char *argv[]) {
    pid_t pid;
    int counter = 0;
    while (counter < 2) {
        pid = fork();
        if (pid > 0) break;
        counter++;
        printf("%d", counter);
    }
    if (counter > 0) printf("%d", counter);
    if (pid > 0) {
        waitpid(pid, NULL, 0);
        counter += 5;
        printf("%d", counter);
    }
    return 0;
}
  • List all possible outputs
  • Possible Output 1: 112265 Possible Output 2: 121265 Possible Output 3: 122165
  • If the > of the counter > 0 test is changed to a >=, then counter values of zeroes would be included in each possible output. How many different outputs are now possible? (No need to list the outputs—just present the number.)
  • Let's go through another example that is the kind of signals problem you may see on the midterm exam.
    • Consider this program and its execution. Assume that all processes run to completion, all system and printf calls succeed, and that all calls to printf are atomic. Assume nothing about scheduling or time slice durations.

Lecture 08: Race Conditions, Deadlock, and Data Integrity

int main(int argc, char *argv[]) {
    pid_t pid;
    int counter = 0;
    while (counter < 2) {
        pid = fork();
        if (pid > 0) break;
        counter++;
        printf("%d", counter);
    }
    if (counter > 0) printf("%d", counter);
    if (pid > 0) {
        waitpid(pid, NULL, 0);
        counter += 5;
        printf("%d", counter);
    }
    return 0;
}
  • List all possible outputs
  • Possible Output 1: 112265 Possible Output 2: 121265 Possible Output 3: 122165
  • If the > of the counter > 0 test is changed to a >=, then counter values of zeroes would be included in each possible output. How many different outputs are now possible? (No need to list the outputs—just present the number.)
  • 18 outputs now (6 x the first number)

Lecture 08: Race Conditions, Deadlock, and Data Integrity

By Chris Gregg

Lecture 08: Race Conditions, Deadlock, and Data Integrity

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