linux-kernel - Null clone CLONE

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Date:	Mon, 17 May 2010 07:29:09 -0700
From:	Earl Chew <echew@...acom.com>
To:	<linux-kernel@...r.kernel.org>
Subject: Null clone CLONE_VM conundrum

I'm looking for some advice to focus my investigation.

I'm using 2.6.31 kernel on PowerPC with glibc version 2.7.

I've been looking into some anomalous behaviour with a program
that uses clone(2).

I've narrowed down the problem to interaction between
the program and the following null clone:

int nullClone(void*)
{
    return 0;
}

...

   pid_t childPid = clone(nullClone, stackPointer,
                          CLONE_VM | SIGCHLD,
                          0, 0, 0, 0);

   waitpid(childPid, &childStatus);

As you can see, the null clone is essentially a nop.

Commenting /* CLONE_VM | */, leaving only SIGCHLD (aka null fork(2))
makes the following problem to go away.

The problem I see is that subsequent to the clone(2):

   pthread_mutex_lock(parentMutex);
     ...
   pthread_mutex_unlock(parentMutex);

   /* Null clone here */

   pthread_mutex_lock(parentMutex);
     ...
   pthread_mutex_unlock(parentMutex);  <---- Gets stuck here.

The mutex in question is created with PTHREAD_PRIO_INHERIT.

There are a few more details regarding null threads which
I won't get into just yet. I need to try to distill the
problem into a smaller program.

I'm suspicious because I believe the null clone should not
have any effect on the caller -- but obviously does, and in
a way I don't understand.

Do you have suggestions as to where I should look next to explain
this anomalous behaviour ?

What effects might the null clone have on the mutex implementation
that I am not accounting for ?

Earl
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