Re: [PATCH v2 10/11] perf/uprobe: Convert single-step and uretprobe to SRCU

[Oleg Nesterov <oleg@redhat.com>]

I'll try to actually apply the whole series and read the code tomorrow.
Right now I can't understand this change... Just one question for now.

On 07/11, Peter Zijlstra wrote:
>
> @@ -1956,11 +1960,13 @@ static void prepare_uretprobe(struct upr
>  			 * attack from user-space.
>  			 */
>  			uprobe_warn(current, "handle tail call");
> -			goto err_uprobe;
> +			goto err_mem;
>  		}
>  		orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
>  	}
>
> +	ri->srcu_idx = __srcu_read_lock(&uretprobes_srcu);
> +	ri->uprobe = uprobe;

It seems that, if we race with _unregister, this __srcu_read_lock()
can happen after call_srcu(uprobes_srcu, uprobe, uprobe_free_stage1)
was already called...

In this case read_lock "has no effect" in that uprobe_free_stage1()
can run before free_ret_instance() does srcu_read_unlock(ri->srcu_idx).

Perhaps it is fine, uprobe_free_stage1() does another call_srcu(),
but somehow I got lost.

Could you re-check this logic? Most probably I missed something, but still...

Oleg.