In the Linux kernel, the following vulnerability has been resolved:

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().