Re: [PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU

[Oleg Nesterov]

On 01/29, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@xxxxxxxxxx> writes:
>
> > With CONFIG_PREEMPT_RCU read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
> > but find_pid_ns()->hlist_for_each_entry_rcu() should be safe under tasklist.
> >
> > Usually it is, detach_pid() is always called under write_lock(tasklist_lock),
> > but copy_process() calls free_pid() lockless.
> >
> > "#ifdef CONFIG_PREEMPT_RCU" is added mostly as documentation, perhaps it is
> > too ugly and should be removed.
> >
> > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> >
> > --- MM/kernel/fork.c~PR_RCU	2008-01-27 17:09:47.000000000 +0300
> > +++ MM/kernel/fork.c	2008-01-29 19:23:44.000000000 +0300
> > @@ -1335,8 +1335,19 @@ static struct task_struct *copy_process(
> >  	return p;
> >
> >  bad_fork_free_pid:
> > -	if (pid != &init_struct_pid)
> > +	if (pid != &init_struct_pid) {
> > +#ifdef CONFIG_PREEMPT_RCU
> > +		/*
> > +		 * read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
> > +		 * make sure find_pid() is safe under read_lock(tasklist).
> > +		 */
> > +		write_lock_irq(&tasklist_lock);
> > +#endif
> >  		free_pid(pid);
> > +#ifdef CONFIG_PREEMPT_RCU
> > +		write_unlock_irq(&tasklist_lock);
> > +#endif
> > +	}
> >  bad_fork_cleanup_namespaces:
> >  	exit_task_namespaces(p);
> >  bad_fork_cleanup_keys:
>
> Ok. I believe I see what problem you are trying to fix.  That
> a pid returned from find_pid might disappear if we are not rcu
> protected.

Not only.

Any find_pid() is unsafe under tasklist, even the find_pid(1).
Because free_pid() mangles the pid_hash[hash] while find_pid()
scans the same list.

> This patch in the simplest form is wrong because it is confusing.
>
> We currently appear to have two options.
> 1) Force all pid hash table access and pid accesses that
>    do not get a count to be covered under rcu_read_lock.

I agree, we can (and should) convert most of these read_lock(tasklist)'s
to rcu_read_lock(). But we have a lot of them.

> 2) To modify the locking requirements for free_pid to require
>    the tasklist_lock.
>
>    However this second approach is horribly brittle, as it
>    will break if we ever have intermediate entries in the
>    hash table protected by pidmap_lock.
>
> Using the tasklist_lock to still guarantee we see the list, the entire
> list, and exactly the list for proper implementation of kill to
> process groups and sessions still seems sane.

And this means that attach_pid() and detach_pid() need write_lock(tasklist)
anyway.

So copy_process()->free_pid() is the only case when we modify the pid_hash[]
list without tasklist.

> So let's just remove the guarantee of find_pid being usable with
> just the tasklist_lock held.
>
> Eric
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index e29a900..0ffb8cc 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -100,8 +100,7 @@ struct pid_namespace;
>  extern struct pid_namespace init_pid_ns;
>
>  /*
> - * look up a PID in the hash table. Must be called with the tasklist_lock
> - * or rcu_read_lock() held.
> + * look up a PID in the hash table. Must be called with the rcu_read_lock() held.

Imho, we should first fix all users of read_lock(tasklist)+find_..._pid().

So I still think this patch makes sense as a trivial fix for now, until
we add the necessary rcu_read_lock()s. However this race is very unlikely,
perhaps we can live with it.

Oleg.

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