[PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition

[Hao Ge <hao.ge@linux.dev>]

From: Hao Ge <gehao@kylinos.cn>

If two competing threads enter alloc_slab_obj_exts(), and the
thread that failed to allocate the object extension vector exits
after the one that succeeded, it will mistakenly assume slab->obj_ext
is still empty due to its own allocation failure. This will then trigger
warnings enforced by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks in
the subsequent free path.

Therefore, let's add an additional check when alloc_slab_obj_exts fails.

Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 mm/slub.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index d4403341c9df..42276f0cc920 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2227,9 +2227,12 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
 	slab = virt_to_slab(p);
 	if (!slab_obj_exts(slab) &&
 	    alloc_slab_obj_exts(slab, s, flags, false)) {
-		pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
-			     __func__, s->name);
-		return NULL;
+		/* Recheck if a racing thread has successfully allocated slab->obj_exts. */
+		if (!slab_obj_exts(slab)) {
+			pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
+				     __func__, s->name);
+			return NULL;
+		}
 	}
 
 	return slab_obj_exts(slab) + obj_to_index(s, slab, p);
-- 
2.25.1