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

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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@xxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
---
   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))		\



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