Question

I’m working on a platform running MontaVista Linux 3.1.

I have a C++ application, which for esoteric reasons which I won’t go into, has to remount the JFFS2 flash file system quite regularly between read-only and read-write.

When you perform a int mount(...) call, specified in sys/mount.h, to set the file system read-write, the jffs2_gcd_mtd0 garbage collector process gets kicked off as you would expect. However, when you repeat the mount call to go back to read-only, jffs2_gcd_mtd0 gets killed, and becomes a defunct process.

After a few minutes, we end up with a shed load of defunct jffs2_gcd_mtd0 processes, which no matter what we do, we can’t get rid of.

I can replicate the problem with the following test app:

int main()
{
  while(true)
  {
    mount("/dev/mtdblock/0", "flash", "", MS_REMOUNT|MS_POSIXACL|MS_ACTIVE|MS_NOUSER|0XEC0000, "");
    sleep(1);
    mount("/dev/mtdblock/0", "flash", "", MS_RDONLY|MS_REMOUNT|MS_POSIXACL|MS_ACTIVE|MS_NOUSER|0XEC0000, "");
    sleep(1);
  }
}

I have tried various method to reap the defunct processes: setting signal(SIGCHLD, SIG_IGN) (doesn’t work); calling wait(int) after the set to read-only (fails, with errno going to 10 – “No child processes”); calling kill(0, SIGCHLD) (doesn’t work).

Am I correct in assuming this is a bug in the mount implementation we have? Given that this is a bug, how could I remove the defunct processes, and stop the process ID table from filling up?

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Some supplementary info: this problem doesn’t seem to occur when I run the test app with strace. Now I’m getting really stumped!

Answer 1

As a workaround, I have found that calling the mount() command from within a pthread allows the defunct jffs2_gcd_mtd0 processes to be reaped.

I believe this is working via the following mechanism: when the thread joins, the spawned jffs2_gcd_mtd0 process is left without a parent. It therefore gets inherited by init, which then gets reaped when it is finished.

If anyone would like to correct/expand on my explanation above, please do!