[ltt-dev] [PATCH 6/7] urcu, call_rcu: Cleanup call_rcu_data pointers before used in child

[Mathieu Desnoyers]

* Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> On Thu, Sep 29, 2011 at 01:59:12PM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> > > On Wed, Sep 28, 2011 at 04:34:32PM +0800, Lai Jiangshan wrote:
> > > > Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> > > > ---
> > > >  urcu-call-rcu-impl.h |    6 ++++++
> > > >  1 files changed, 6 insertions(+), 0 deletions(-)
> > > > 
> > > > diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
> > > > index 65c1c7a..3c68ae7 100644
> > > > --- a/urcu-call-rcu-impl.h
> > > > +++ b/urcu-call-rcu-impl.h
> > > > @@ -686,6 +686,12 @@ void call_rcu_after_fork_child(void)
> > > >  	default_call_rcu_data = NULL;
> > > >  	(void)get_default_call_rcu_data();
> > > > 
> > > > +	/* Cleanup call_rcu_data pointers before used */
> > > > +	maxcpus = 0;
> > > > +	free(per_cpu_call_rcu_data);
> > > > +	per_cpu_call_rcu_data = NULL;
> > > 
> > > Good catch!  I would expect that the number of CPUs would be the
> > > same for the child as it was for the parent, but doesn't hurt to
> > > make the child start over.
> > > 
> > > > +	thread_call_rcu_data = NULL;
> > > 
> > > Isn't thread_call_rcu_data already NULL due to the child being a new
> > > thread and the C initialization-to-zero rules?
> > 
> > #include <unistd.h>
> > #include <stdio.h>
> > 
> > int __thread a;
> > 
> > int main()
> > {
> > 	 int pid;
> > 
> > 	 a = 1;
> > 	 pid = fork();
> > 	 if (pid > 0) {
> > 		  printf("parent val %d\n", a);
> > 		  return 0;
> > 	 } else {
> > 		  printf("child val %d\n", a);
> > 		  return 0;
> > 	 }
> > }
> > 
> > compudj at thinkos:/tmp$ gcc -o test test.c
> > compudj at thinkos:/tmp$ ./test
> > parent val 1
> > child val 1
> > 
> > AFAIK, the c initialization to zero rules apply to exec(), not fork.
> > Here we are taking care of after-fork in the child, in cases where the
> > child is not doing an exec.
> > 
> > Does that make more sense ?
> 
> I would think that the forked child would be a new thread, but obviously
> not!  I stand corrected.

It all makes more sense when you consider the following paragraph of
pthread_atfork(3):

       To understand the purpose of pthread_atfork, recall that fork(2) dupli‐
       cates the whole memory space, including mutexes in their current  lock‐
       ing  state,  but only the calling thread: other threads are not running
       in the child process.  The mutexes are not usable after  the  fork  and
       must be initialized with pthread_mutex_init in the child process.  This
       is a limitation of the current implementation and might or might not be
       present in future versions.

Thanks,

Mathieu

> 
> 							Thanx, Paul
> 
> > Thanks,
> > 
> > Mathieu
> > 
> > 
> > > 
> > > 							Thanx, Paul
> > > 
> > > > +
> > > >  	/* Dispose of all of the rest of the call_rcu_data structures. */
> > > >  	cds_list_for_each_entry_safe(crdp, next, &call_rcu_data_list, list) {
> > > >  		if (crdp == default_call_rcu_data)
> > > > -- 
> > > > 1.7.4.4
> > > > 
> > 
> > -- 
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com
> > 
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com