• Anders Kaseorg's avatar
    fifo: Do not restart open() if it already found a partner · 05d290d6
    Anders Kaseorg authored
    If a parent and child process open the two ends of a fifo, and the
    child immediately exits, the parent may receive a SIGCHLD before its
    open() returns.  In that case, we need to make sure that open() will
    return successfully after the SIGCHLD handler returns, instead of
    throwing EINTR or being restarted.  Otherwise, the restarted open()
    would incorrectly wait for a second partner on the other end.
    The following test demonstrates the EINTR that was wrongly thrown from
    the parent’s open().  Change .sa_flags = 0 to .sa_flags = SA_RESTART
    to see a deadlock instead, in which the restarted open() waits for a
    second reader that will never come.  (On my systems, this happens
    pretty reliably within about 5 to 500 iterations.  Others report that
    it manages to loop ~forever sometimes; YMMV.)
      #include <sys/stat.h>
      #include <sys/types.h>
      #include <sys/wait.h>
      #include <fcntl.h>
      #include <signal.h>
      #include <stdio.h>
      #include <stdlib.h>
      #include <unistd.h>
      #define CHECK(x) do if ((x) == -1) {perror(#x); abort();} while(0)
      void handler(int signum) {}
      int main()
          struct sigaction act = {.sa_handler = handler, .sa_flags = 0};
          CHECK(sigaction(SIGCHLD, &act, NULL));
          CHECK(mknod("fifo", S_IFIFO | S_IRWXU, 0));
          for (;;) {
              int fd;
              pid_t pid;
              putc('.', stderr);
              CHECK(pid = fork());
              if (pid == 0) {
                  CHECK(fd = open("fifo", O_RDONLY));
              CHECK(fd = open("fifo", O_WRONLY));
              CHECK(waitpid(pid, NULL, 0));
    This is what I suspect was causing the Git test suite to fail in
    	http://bugs.debian.org/678852Signed-off-by: Anders Kaseorg's avatarAnders Kaseorg <andersk@mit.edu>
    Reviewed-by: 's avatarJonathan Nieder <jrnieder@gmail.com>
    Cc: stable@kernel.org
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
fifo.c 3.18 KB