Re: [PATCH v2 1/1] tracing: fgraph: allocate shadow stacks inline with GFP_NOWAIT

[Vineet Gupta <vineet.gupta@linux.dev>]

On 9/28/26 6:03 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Low] cleanup: mixing goto-based error handling and scope-based cleanup
> --
>
> commit a6de528fef00ef67da15965a7a8b3944fa92b053
> Author: Vineet Gupta <vineet.gupta@linux.dev>
> Subject: tracing: fgraph: allocate shadow stacks inline with GFP_NOWAIT
>
> This commit aims to resolve performance regressions with ftrace graph tracing
> when there are a large number of tasks. It replaces the pre-allocated batch
> approach with inline allocation using GFP_NOWAIT, avoiding repeated sweeps of
> the task list. Additionally, it transitions the RCU read-side section to use
> scoped_guard(rcu) instead of manual rcu_read_lock and rcu_read_unlock calls.
>
>> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
>> index 40d373d65f9b9..1c5ccc3f80d53 100644
>> --- a/kernel/trace/fgraph.c
>> +++ b/kernel/trace/fgraph.c
> [ ... ]
>> @@ -1057,26 +1056,37 @@ static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
>>   		}
>>   	}
>>   
>> -	rcu_read_lock();
>> -	for_each_process_thread(g, t) {
>> -		if (start == end) {
>> -			ret = -EAGAIN;
>> -			goto unlock;
>> -		}
>> +	scoped_guard (rcu) {
> [Severity: Low]
> Does this mix goto-based error handling and scope-based cleanup within the same
> function?
>
> Looking at alloc_retstack_tasklist(), the function retains the legacy goto free
> path in the initial pre-allocation loop (just before this block), while
> introducing scoped_guard(rcu) here.
>
> The cleanup subsystem guidelines recommend that for a given routine, we should
> convert all resources that need a goto cleanup to scope-based cleanup, or
> convert none of them. Mixing these paradigms creates confusing ownership
> semantics and is highly prone to double-free or resource leak regressions upon
> future modifications.
>
> Could we either convert the ret_stack_list array cleanup to use scope-based
> helpers, or stick to the manual rcu_read_lock() and rcu_read_unlock() here to
> keep the error handling style consistent?

And a third option is to drop the fallback allocation completely and 
return -EAGAIN and come back. Granted I don't have much experience of 
typical handling of GFP_NOWAIT fails and retrying immediately: would 
that recover at all or does that take us back to where we started ? Peter

Thx,
-Vineet