radix-tree: fix kmemleak false positives on tree head reassignment

Kmemleak periodically reports transient false positives for radix tree
nodes allocated through the IDR, for example:

  unreferenced object 0xffff0004d6ac4200 (size 576):
    comm "tcpeventd", pid 6412
    backtrace (crc 335d668a):
      kmem_cache_alloc_noprof
      radix_tree_node_alloc
      radix_tree_extend
      idr_get_free
      idr_alloc_cyclic
      map_create
      __sys_bpf

radix_tree_extend() (grow) and radix_tree_shrink() (shrink) repoint
root->xa_head to a new node.  If a kmemleak scan has already walked past
root->xa_head, the new head is not reachable from any scanned pointer
until the following scan, so kmemleak reports it as leaked even though it
is live.

This is the same race fixed for the XArray API in commit a1a029bcea59
("XArray: fix kmemleak false positive in xas_shrink()").  The IDR uses the
radix tree API directly and hits it on both the grow and the shrink path,
so mark the new head as a transient leak in both.

Add a matching kmemleak_transient_leak() stub to the radix tree test
harness so the userspace lib/radix-tree.c build keeps building.

Link: https://lore.kernel.org/20260703-radix-tree-v2-1-38bb6efb5f6e@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/tools/testing/shared/linux/kmemleak.h b/tools/testing/shared/linux/kmemleak.h
index 155f112786c4..65a383090fb5 100644
--- a/tools/testing/shared/linux/kmemleak.h
+++ b/tools/testing/shared/linux/kmemleak.h
@@ -1 +1,2 @@
 static inline void kmemleak_update_trace(const void *ptr) { }
+static inline void kmemleak_transient_leak(const void *ptr) { }