Reproducible, long-standing fanotify+autofs problem

[Marko Rauhamaa <marko.rauhamaa@f-secure.com>]

We have run into an easily reproducible fanotify hang that affects
several distros as well as the latest development kernel. (It isn't
fixed by Amir Goldstein's recent fanotify patch, BTW.)

Here is a simple reproduction:

 1. Compile and build the tiny test program below, which has been
    adapted from the example in "man fanotify":

    cc -o fantest3 fantest.c

 2. Install autofs.

 3. Run these commands as root:

      mkdir -p foo/home/test foo/user/test
      echo /sshome /etc/auto.sshome >/etc/auto.master
      echo "test / :/root/foo/home/test /user :/root/foo/user/test" \
          >/etc/auto.sshome
      systemctl start autofs
      ls -l /sshome/test
      ./fantest3 / /sshome/test /sshome/test/user/

      # NB! The final slash in the fantest3 command can be significant!

  4. Observe the last command hang. While it is hung, other file system
     operations from other processes are blocked.

  5. Press Ctrl-C to force fantest3 to exit. The file system becomes
     alive. Rerunning the fantest3 command no longer hangs.

  6. To get back into the problem state, execute (as root):

      systemctl restart autofs
      ls -l /sshome/test
      ./fantest3 / /sshome/test /sshome/test/user/


===Begin fantest.c======================================================
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/fanotify.h>

int main(int argc, char *argv[])
{
    int fd, i;
    if (argc < 2) {
        fprintf(stderr, "Usage: %s MOUNT ...\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK,
                       O_RDONLY);
    if (fd == -1) {
        perror("fanotify_init");
        exit(EXIT_FAILURE);
    }
    for (i = 1; i < argc; i++)
        if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT,
                          FAN_OPEN_PERM | FAN_CLOSE_WRITE, AT_FDCWD,
                          argv[i]) == -1) {
            perror("fanotify_mark");
            exit(EXIT_FAILURE);
        }
    exit(EXIT_SUCCESS);
}
===End fantest.c========================================================


Marko