Non-existing p_vmspace. When is it possible?

[Kostik Belousov]

On Wed, Mar 11, 2009 at 04:38:39PM +0300, pluknet wrote:
> Hi.
> 
> I perform in FOREACH_PROC_IN_SYSTEM(), where I conditionally
> look at p_vmspace internals. I'd like to know the safe way to
> reference p_vmspace fields without potential null-dereference.
> 
> I see an example in vm_pageout_oom(), where making access to
> p->p_vmspace fields is done without additional checks.
> Nevertheless I want to further catch on how it works.
> 
> Currently I additionally explicitly check on P_SYSTEM and PRS_NEW,
> then p->p_vmspace against NULL.
> 
> So I'd wish to understand if a time-window between
> 1) placing a new process to proclist
>  and
> 2) attaching vmspace to this process
> is possible at all, and then in what cases.
> 
> I see in fork1() that a new process' (named p2 here) state is set to
> PRS_NEW just before LIST_INSERT_HEAD(&allproc, p2, p_list) and then
> (after vmspace is already attached in vm_forkproc()) is set to PRS_NORMAL.
> 
> So an additional check on p_vmspace != NULL is not need.
> Am I right?

The canonical sequence of doing this is, assuming p is a held pointer
to a process:

	vm = vmspace_acquire_ref(p);
	if (vm == NULL) {
		PRELE(p);
		return ?;
	}
	use vm;
	vmspace_free(vm);

Look around the tree for the vmspace_acquire_ref usage.
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