If the last cmpxchg() done when increasing the tree depth fails
(because another caller has done a concurrent update) the 'new_node'
is reused later for either a array of pointers or the user data.
However the new_node->children[0] has been set non-zero this will
either end up being treated as a pointer to another node or as user data.
Rather than just setting children[0] = NULL free back the unwanted page
in the very unusual case that the cmpxchg() fails.
This removes quite a few comparisons from the normal path.
Signed-off-by: David Laight <david.laight@xxxxxxxxxx>
---
Although rather nasty not really worth a backport because neither of the
2 places this code is used can possibly hit the failing path.
lib/generic-radix-tree.c | 37 +++++++++++++++++--------------------
1 file changed, 17 insertions(+), 20 deletions(-)
diff --git a/lib/generic-radix-tree.c b/lib/generic-radix-tree.c
index f25eb111c051..5695fe547f9d 100644
--- a/lib/generic-radix-tree.c
+++ b/lib/generic-radix-tree.c
@@ -105,7 +105,7 @@ void *__genradix_ptr_alloc(struct __genradix *radix, size_t offset,
gfp_t gfp_mask)
{
struct genradix_root *v = READ_ONCE(radix->root);
- struct genradix_node *n, *new_node = NULL;
+ struct genradix_node *n, *new_node;
unsigned level;
/* Increase tree depth if necessary: */
@@ -118,20 +118,19 @@ void *__genradix_ptr_alloc(struct __genradix *radix, size_t offset,
if (n && ilog2(offset) < genradix_depth_shift(level))
break;
- if (!new_node) {
- new_node = genradix_alloc_node(gfp_mask);
- if (!new_node)
- return NULL;
- }
+ new_node = genradix_alloc_node(gfp_mask);
+ if (!new_node)
+ return NULL;
new_node->children[0] = n;
new_root = ((struct genradix_root *)
((unsigned long) new_node | (n ? level + 1 : 0)));
- if ((v = cmpxchg_release(&radix->root, r, new_root)) == r) {
+ v = cmpxchg_release(&radix->root, r, new_root);
+ if (v == r)
v = new_root;
- new_node = NULL;
- }
+ else
+ genradix_free_node(new_node);
}
while (level--) {
@@ -141,20 +140,18 @@ void *__genradix_ptr_alloc(struct __genradix *radix, size_t offset,
n = READ_ONCE(*p);
if (!n) {
- if (!new_node) {
- new_node = genradix_alloc_node(gfp_mask);
- if (!new_node)
- return NULL;
- }
-
- if (!(n = cmpxchg_release(p, NULL, new_node)))
- swap(n, new_node);
+ new_node = genradix_alloc_node(gfp_mask);
+ if (!new_node)
+ return NULL;
+
+ n = cmpxchg_release(p, NULL, new_node);
+ if (!n)
+ n = new_node;
+ else
+ genradix_free_node(new_node);
}
}
- if (new_node)
- genradix_free_node(new_node);
-
return &n->data[offset];
}
EXPORT_SYMBOL(__genradix_ptr_alloc);
--
2.25.1
-
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