[Mesa-dev] [PATCH 1/2] util: Fix foreach_list_typed_safe when exec_node is not at offset 0.

[Connor Abbott]

On Mon, Mar 9, 2015 at 10:35 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Mon, Mar 9, 2015 at 7:24 PM, Matt Turner <mattst88 at gmail.com> wrote:
>> On Mon, Mar 9, 2015 at 6:36 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>>> From: Jason Ekstrand <jason.ekstrand at intel.com>
>>>
>>> __next and __prev are pointers to the structure containing the exec_node
>>> link, not the embedded exec_node.  NULL checks would fail unless the
>>> embedded exec_node happened to be at offset 0 in the parent struct.
>>>
>>> Signed-off-by: Jason Ekstrand <jason.ekstrand at intel.com>
>>> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
>>> ---
>>>  src/glsl/list.h | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/src/glsl/list.h b/src/glsl/list.h
>>> index ddb98f7..680e963 100644
>>> --- a/src/glsl/list.h
>>> +++ b/src/glsl/list.h
>>> @@ -684,7 +684,7 @@ inline void exec_node::insert_before(exec_list *before)
>>>             exec_node_data(__type, (__list)->head, __field),                \
>>>                 * __next =                                                  \
>>>             exec_node_data(__type, (__node)->__field.next, __field);        \
>>> -        __next != NULL;                                                    \
>>> +        &__next->__field != NULL;                                          \
>>
>> I'm not understanding now the address of __next->__field can ever be NULL.
>>
>> __next is something with an embedded struct exec_node, so don't we
>> want "__next->__field != NULL" without the address-of operator?
>
> Sorry, that should have been "exec_node_is_tail_sentinel(&__next->__field)"

No, that won't work. We want to bail out if the *current* node is the
tail sentinel, not the next one. If the next node is the tail
sentinel, then the current one is the last element, so we have to go
through the loop once more. We could use exec_node_is_tail_sentinel()
on the current node, but we've already dereferenced the pointer
earlier when getting the next node so it would be less efficient to
dereference it again.

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