perf/core: Fix a refcount leak in attach_perf_ctx_data()

The attach_perf_ctx_data() can race on global and !global cases.  The
global case is protected by global_ctx_data_rwsem and shares a single
reference count using perf_ctx_data.global field.

But when it races with !global case, it may miss to set the global field
and result in a reference count leak.

 CPU1                                  CPU2
 ----------------------------------------------------------------
 attach_task_ctx_data(.global=1)       attach_task_ctx_data(.global=0)
   cd1 = alloc_perf_ctx_data();          cd2 = alloc_perf_ctx_data();
                                         //    { .global = 0, .refcount = 1 };

                                         try_cmpxchg(); // success,
                                         // task->perf_ctx_data = cd2
   try_cmpxhg(); // fail; old = cd2
   refcount_inc_not_zero(&old->refcount); // success
     // old.refcount = 2
   free_perf_ctx_data(cd1);

Then later detach_global_ctx_data() will see the data but it's not
marked as global, so it won't call detach_task_ctx_data().

Fixes: 506e64e710 ("perf: attach/detach PMU specific data")
Assisted-by: Sashiko.dev:Gemini-3.1-pro
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260920231639.11910-1-namhyung@kernel.org

kernel/events/core.c

@@ -5454,6 +5454,8 @@ attach_task_ctx_data(struct task_struct *task, struct kmem_cache *ctx_cache,
 		}
 
 		if (refcount_inc_not_zero(&old->refcount)) {
+			if (global)
+				old->global = true;
 			free_perf_ctx_data(cd); /* unused */
 			return 0;
 		}