Date:	Wed, 25 Nov 2015 15:06:32 +0100
From:	Ilya Dryomov <idryomov@...il.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	SF Markus Elfring <elfring@...rs.sourceforge.net>,
	Alex Elder <elder@...nel.org>, Sage Weil <sage@...hat.com>,
	Ceph Development <ceph-devel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org,
	Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH 2/2] block-rbd: One function call less in
 rbd_dev_probe_parent() after error detection

On Wed, Nov 25, 2015 at 12:55 PM, Dan Carpenter
<dan.carpenter@...cle.com> wrote:
> On Tue, Nov 24, 2015 at 09:21:06PM +0100, Ilya Dryomov wrote:
>> >> Cleanup here is (and should be) done in reverse order.
>> >
>
> Yes.  This is true.
>
>> > I have got an other impression about the appropriate order for the corresponding
>> > clean-up function calls.
>> >
>> >
>> >> We allocate parent rbd_device and then link it with what we already have,
>> >
>> > I guess that we have got a different understanding about the relevant "linking".
>>
>> Well, there isn't any _literal_ linking (e.g. adding to a link list,
>> etc) in this case.  We just bump some refs and do probe to fill in the
>> newly allocated parent.  If probe fails, we put refs and free parent,
>> reversing the "alloc parent, bump refs" order.
>>
>> The actual linking (rbd_dev->parent = parent) is done right before
>> returning so we never have to undo it in rbd_dev_probe_parent() and
>> that's the only reason your patch probably doesn't break anything.
>> Think about what happens if, after your patch is applied, someone moves
>> that assignment up or adds an extra step that can fail after it...
>>
>
> The problem is that the unwind code should be a mirror of the allocate
> code but rbd_dev_unparent() doesn't mirror anything.  Generally, writing
> future proof stubs like this is a wrong thing because predicting the
> future is hard and in the mean time we are left stubs which confuse
> everyone.

It's not a future proof stub.  It's just some crufty code that was
fixed over time to not leak things.  I won't defend it - it is
confusing and could definitely be improved - but that can't be done
without refactoring a fair bunch of calling code.  A patch changing
rbd_dev_probe_parent() alone just won't do it.

>
>> If all error paths could be adjusted so that NULL pointers are never
>> passed in, destroy functions wouldn't need to have a NULL check, would
>> they?
>
> Yep.  We agree on the right way to do it.  I am probably the number one
> kernel developer for removing the most sanity checks.  :P  (As opposed
> to patch 1/1 where we now rely on the sanity check inside
> rbd_dev_destroy().)
>
> drivers/block/rbd.c
>   5149  static int rbd_dev_probe_parent(struct rbd_device *rbd_dev, int depth)
>   5150  {
>   5151          struct rbd_device *parent = NULL;
>   5152          int ret;
>   5153
>   5154          if (!rbd_dev->parent_spec)
>   5155                  return 0;
>   5156
>   5157          if (++depth > RBD_MAX_PARENT_CHAIN_LEN) {
>   5158                  pr_info("parent chain is too long (%d)\n", depth);
>   5159                  ret = -EINVAL;
>   5160                  goto out_err;
>
> We haven't allocated anything so this should just be return -EINVAL;
> In the original code, we decrement the kref count on ->parent_spec on
> this error path so that is a classic One Err Bug.

The caller expects rbd_dev->parent_spec to be put on any error.  Notice
that we return right away if !rbd_dev->parent_spec.

>
>   5161          }
>   5162
>   5163          parent = rbd_dev_create(rbd_dev->rbd_client, rbd_dev->parent_spec,
>   5164                                  NULL);
>   5165          if (!parent) {
>   5166                  ret = -ENOMEM;
>   5167                  goto out_err;
>
> Still haven't allocated anything so return -ENOMEM, but if we fail after
> this point we will need to call rbd_dev_destroy().
>
>   5168          }
>   5169
>   5170          /*
>   5171           * Images related by parent/child relationships always share
>   5172           * rbd_client and spec/parent_spec, so bump their refcounts.
>   5173           */
>   5174          __rbd_get_client(rbd_dev->rbd_client);
>   5175          rbd_spec_get(rbd_dev->parent_spec);
>
> We will need to put these on any later error paths.

And we do, in rbd_dev_destroy(parent), since these are references for
the parent.

>
>   5176
>   5177          ret = rbd_dev_image_probe(parent, depth);
>   5178          if (ret < 0)
>   5179                  goto out_err;
>
> Ok.  We need to put the ->parent_spec, ->rbd_client and free the parent.
>
>   5180
>   5181          rbd_dev->parent = parent;
>   5182          atomic_set(&rbd_dev->parent_ref, 1);
>   5183          return 0;
>   5184
>   5185  out_err:
>   5186          rbd_dev_unparent(rbd_dev);
>
> This is a complicated way to say rbd_spec_put(rbd_dev->parent_spec);
>
> Also, is it really necessary to set ->parent_spec to NULL?  If we didn't
> put the last reference then doesn't setting it to NULL mean we are
> leaking?  Setting it to NULL is confusing and feels like a layering
> violation.

Yes, because as it is, ->parent_spec is a determinant of whether or not
the image has a parent.  If we fail in rbd_dev_probe_parent(), it needs
to be set to NULL to signify that the image doesn't have a parent.
Even if the entire thing was refactored, we'd still have to do the same
because not every image has a parent and the same error path has to
work for all images.  The layering violation is that we have to do in
rbd_dev_probe_parent() even though we didn't allocate it there.

>
>   5187          if (parent)
>   5188                  rbd_dev_destroy(parent);
>   5189          return ret;
>   5190  }
>
> I feel like we should be calling rbd_put_client() on this error path or
> else the code is buggy or has layer violations.  So I *think* it should
> look like this:
>
> dec_ref_counts:
>         rbd_spec_put(rbd_dev->parent_spec);
>         rbd_put_client(rbd_dev->rbd_client);
>
>         rbd_dev_destroy(parent);
>
>         return ret;

We do, in rbd_dev_destroy(parent).

Thanks,

                Ilya
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