Re: [PATCH v11 06/14] unwind_user/deferred: Add deferred unwinding interface

[Steven Rostedt]

On Wed, 25 Jun 2025 18:56:06 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

>  static __always_inline void unwind_reset_info(void)
>  {
> -	if (unlikely(current->unwind_info.cache))
> +	/* Exit out early if this was never used */
> +	if (likely(!current->unwind_info.timestamp))
> +		return;

I found that this breaks the use of perf using the unwind_user_faultable()
directly and not relying on the deferred infrastructure (which it does when
it traces a single task and also needs to remove the separate in_nmi()
code). Because this still requires the nr_entries to be set to zero.

The clearing of the nr_entries has to be separate from the timestamp. To
prevent unneeded writes after the cache is allocated, should we check the
nr_entries is set before writing zero?

	if (current->unwind_info.cache && current->unwind_info.cache->nr_entries)
  		current->unwind_info.cache->nr_entries = 0;

?

-- Steve

> +
> +	if (current->unwind_info.cache)
>  		current->unwind_info.cache->nr_entries = 0;
> +	current->unwind_info.timestamp = 0;
>  }