Date:	Mon, 14 Feb 2011 22:43:47 -0800
From:	"Kenneth Albanowski (Palm GBU)" <Kenneth.Albanowski@...m.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Question about clearing of tsk->robust_list in clone

I've been tracking down a bug I ran into with a robust pthreads mutex
shared between a parent and a child that it forked. This finally came
down to a bad interaction between glibc and the kernel (and looks to
be present in the current Linux trees as well as glibc 2.13): 
copy_process() explicitly clears the robust_list pointer in the cloned
child. However, there is currently no logic in nptl to re-establish
the robust list pointer after a fork.

There was some conversation about this in the Fedora bugtracker:

  https://bugzilla.redhat.com/show_bug.cgi?id=628608

Those folks appear to have reached the same conclusion as I: this
could either be solved with some potentially complex glibc code, or by
simply not having the kernel NULL out robust_list in the child.

That code came in at:

    commit 8f17d3a5049d32392b79925c73a0cf99ce6d5af0
    Author: Ingo Molnar <mingo@...e.hu>
    Date:   Mon Mar 27 01:16:27 2006 -0800

        [PATCH] lightweight robust futexes updates
    
        - fix: initialize the robust list(s) to NULL in copy_process.
    
        - doc update
    
        - cleanup: rename _inuser to _inatomic
    
        - __user cleanups and other small cleanups
    
Can anyone say what problem was being fixed by initializing the robust
list(s) to NULL? I've stared at the implementation, and I cannot see any
harm (potentially a slight bit more work in exec, but no harm) in not
clearing them.

Test program appended, to demonstrate the issue somewhat concisely.

- Kenneth

-- child_robust_mutex_death.c --
#define _GNU_SOURCE // for ROBUST mutexes from pthread.h
#include <sys/mman.h>
#include <sys/wait.h>
#include <pthread.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>

int main()
{
  // Use an anonymous shared mapping to put the same mutex in child as in parent. Any other
  // sharing technique would work, this is simply easy and doesn't clutter.
  void * region = mmap(NULL, 4096, PROT_READ|PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS, -1, 0);
  pthread_mutex_t * mutex = (pthread_mutex_t*)region;

  pthread_mutexattr_t attr;

  pthread_mutexattr_init(&attr);
  pthread_mutexattr_setrobust_np(&attr, PTHREAD_MUTEX_ROBUST_NP);
  pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
  pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);
  pthread_mutex_init(mutex, &attr); 

  if (fork() == 0) {
    // child
    // To demonstrate issue, this lock must occur directly in a fork child which has not exec'd, and
    // not in a thread created via pthread_create after forking.
    pthread_mutex_lock(mutex);

    // We now exit from the child, which should trigger the robust mechanism to clean up the mutex.
    _exit(0);
    
  } else {
    // parent
    wait(NULL); // wait for child to die.
    
    // try to obtain the lock, indicating whether the child still has it. This test could be
    // done from any process with access to the mutex, it is merely convenient to do it from
    // the parent.
      
    int err = pthread_mutex_trylock(mutex);
    if (err == 0) {
      printf("Failed, mutex unlocked, but no EOWNERDEAD -- I don't know what happened.\n");
    } else if (err == EBUSY) {
      printf("Failed, mutex not unlocked -- fork child tsk->robust_list == NULL issue.\n");
    } else if (err == EOWNERDEAD) {
      printf("Success, robust mutex reported owner death as intended.\n");
    }
    _exit(0);
  }
}

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