Re: [Intel-gfx] [PATCH v2 1/3] drm/mm: Ensure that the entry is not NULL before extracting rb_node

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 18/02/2022 03:47, Kasireddy, Vivek wrote:
> Hi Tvrtko,
> 
>>
>> On 17/02/2022 07:50, Vivek Kasireddy wrote:
>>> While looking for next holes suitable for an allocation, although,
>>> it is highly unlikely, make sure that the DECLARE_NEXT_HOLE_ADDR
>>> macro is using a valid node before it extracts the rb_node from it.
>>
>> Was the need for this just a consequence of insufficient locking in the
>> i915 patch?
> [Kasireddy, Vivek] Partly, yes; but I figured since we are anyway doing
> if (!entry || ..), it makes sense to dereference entry and extract the rb_node
> after this check.

Unless I am blind I don't see that it makes a difference. 
"&entry->rb_hole_addr" is taking an address of, which works "fine" is 
entry is NULL. And does not get past the !entry check for the actual 
de-reference via RB_EMPTY_NODE. With your patch you move that after the 
!entry check but still have it in the RB_EMPTY_NODE macro. Again, unless 
I am blind, I think just drop this patch.

Regards,

Tvrtko


> Thanks,
> Vivek
> 
>>
>> Regards,
>>
>> Tvrtko
>>
>>>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
>>> Cc: Christian König <christian.koenig@amd.com>
>>> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
>>> ---
>>>    drivers/gpu/drm/drm_mm.c | 5 +++--
>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c
>>> index 8257f9d4f619..499d8874e4ed 100644
>>> --- a/drivers/gpu/drm/drm_mm.c
>>> +++ b/drivers/gpu/drm/drm_mm.c
>>> @@ -389,11 +389,12 @@ first_hole(struct drm_mm *mm,
>>>    #define DECLARE_NEXT_HOLE_ADDR(name, first, last)			\
>>>    static struct drm_mm_node *name(struct drm_mm_node *entry, u64 size)	\
>>>    {									\
>>> -	struct rb_node *parent, *node = &entry->rb_hole_addr;		\
>>> +	struct rb_node *parent, *node;					\
>>>    									\
>>> -	if (!entry || RB_EMPTY_NODE(node))				\
>>> +	if (!entry || RB_EMPTY_NODE(&entry->rb_hole_addr))		\
>>>    		return NULL;						\
>>>    									\
>>> +	node = &entry->rb_hole_addr;					\
>>>    	if (usable_hole_addr(node->first, size)) {			\
>>>    		node = node->first;					\
>>>    		while (usable_hole_addr(node->last, size))		\