[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 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 ?

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