Re: [PATCH RESEND] perf/core: Fix a refcount leak in attach_perf_ctx_data()

[Peter Zijlstra]

On Sun, Sep 20, 2026 at 04:16:39PM -0700, Namhyung Kim wrote:
> 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()
>                                        try_cmpxchg() // ok
>                                        // task->perf_ctx_data = cd2
> 
>      try_cmpxchg()            // fail; old = cd2; global = 0
>      refcount_inc_not_zero()  // cd2->refcount++;
>      free_perf_ctx_data()     // cd1

Urgh, took a good while to remember how all that worked. Also, I think
it might have been clearer written like so:

  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: 506e64e710ff ("perf: attach/detach PMU specific data")
> Assisted-by: Sashiko.dev:Gemini-3.1-pro
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  kernel/events/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index db7b76d6b68aa55d..e180134bad5e0ae3 100644
> --- a/kernel/events/core.c
> +++ b/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;
>  		}