Re: [PATCH v13 10/14] unwind: Clear unwind_mask on exit back to user space

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 15 Jul 2025 12:29:12 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> The below is the last four patches rolled into one. Not been near a
> compiler.

And it shows ;-)

> @@ -117,13 +138,13 @@ static void unwind_deferred_task_work(st
>  	struct unwind_task_info *info = container_of(head, struct unwind_task_info, work);
>  	struct unwind_stacktrace trace;
>  	struct unwind_work *work;
> +	unsigned long bits;
>  	u64 cookie;
>  
> -	if (WARN_ON_ONCE(!info->pending))
> +	if (WARN_ON_ONCE(!unwind_pending(info)))
>  		return;
>  
> -	/* Allow work to come in again */
> -	WRITE_ONCE(info->pending, 0);
> +	bits = atomic_long_fetch_andnot(UNWIND_PENDING, &info->unwind_mask);

I may need to do what other parts of the kernel has done and turn the
above into:

	bits = atomic_long_fetch_andnot(UNWIND_PENDING, (atomic_long_t *)&info->unwind_mask);

As there's other bit manipulations that atomic_long does not take care
of and it's making the code more confusing. When I looked to see how
other users of atomic_long_andnot() did things, most just typecasted
the value to use that function :-/

-- Steve


>  
>  	/*
>  	 * From here on out, the callback must always be called, even if it's
> @@ -136,9 +157,11 @@ static void unwind_deferred_task_work(st
>  
>  	cookie = info->id.id;
>  
> -	guard(mutex)(&callback_mutex);
> -	list_for_each_entry(work, &callbacks, list) {
> -		work->func(work, &trace, cookie);
> +	guard(srcu_lite)(&unwind_srcu);
> +	list_for_each_entry_srcu(work, &callbacks, list,
> +				 srcu_read_lock_held(&unwind_srcu)) {
> +		if (test_bit(work->bit, &bits))
> +			work->func(work, &trace, cookie);
>  	}
>  }
>  
> @@ -162,7 +185,7 @@ static void unwind_deferred_task_work(st
>   * because it has already been previously called for the same entry context,
>   * it will be called again with the same stack trace and cookie.
>   *
> - * Return: 1 if the the callback was already queued.
> + * Return: 1 if the callback was already queued.
>   *         0 if the callback successfully was queued.
>   *         Negative if there's an error.
>   *         @cookie holds the cookie of the first request by any user