Re: [PATCH v2 2/2] pid: Remove pidhash

Oleg Nesterov
2017-09-27T16:28:14Z

If I was not clear...

in short, after this patch the very first idr_alloc_cyclic() is already
wrong. Because, once again, the new not-fully-initialized pid can be found
by find_pid_ns().

perhaps you should chane the previous patch to do
idr_alloc_cyclic(ptr = NULL) and use idr_replace() in this patch after
the PIDNS_HASH_ADDING check.

And I just noticed you didn't cc Eric Biederman <ebiederm@xmission.com>,
please do next time.

On 09/27, Oleg Nesterov wrote:
>
> On 09/27, Gargi Sharma wrote:
> >
> > -#define find_next_offset(map, off) \
> > - find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
> > -
>
> this should go into the previous patch, but this is minor...
>
> > @@ -208,12 +200,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> >
> > upid = pid->numbers + ns->level;
> > spin_lock_irq(&pidmap_lock);
> > - if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
> > + if (!(ns->pid_allocated & PIDNS_ADDING))
> > goto out_unlock;
> > for ( ; upid >= pid->numbers; --upid) {
> > - hlist_add_head_rcu(&upid->pid_chain,
> > - &pid_hash[pid_hashfn(upid->nr, upid->ns)]);
> > - upid->ns->nr_hashed++;
> > + upid->ns->pid_allocated++;
>
> No, this is wrong.
>
> It is too late to check PIDNS_HASH_ADDING/PIDNS_ADDING and increment pid_allocated,
> once we call idr_alloc_cyclic() this pid is already "hashed" in that it can be found
> by find_pid_ns() with this patch applied.
>
> And of course, it is too late to do atomic_set(&pid->count, 1) and initialize
> pid->tasks[type] lists by the same reason.
>
> Oleg.