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

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

On 9/29/26 4:46 PM, bot+bpf-ci@kernel.org wrote:
>> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
>> index ed455b53513b..336f40dc6618 100644
>> --- a/kernel/trace/fgraph.c
>> +++ b/kernel/trace/fgraph.c
>> @@ -1036,10 +1036,9 @@ trace_func_graph_ent_t ftrace_graph_entry = ftrace_graph_entry_stub;
>>   /* Try to assign a return stack array on FTRACE_RETSTACK_ALLOC_SIZE tasks. */
>>   static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
>>   {
> Does this comment still match the function's behavior? Before the patch,
> alloc_retstack_tasklist() assigned at most FTRACE_RETSTACK_ALLOC_SIZE
> stacks per call and returned -EAGAIN to trigger another batch. After the
> patch, it walks every thread in one pass, allocating inline with
> GFP_NOWAIT. The FTRACE_RETSTACK_ALLOC_SIZE pre-allocated entries are now
> only a fallback when GFP_NOWAIT fails, and in the normal case they're all
> freed at the free: label.

Posted v3 [1] with comment fix.

Thx,
-Vineet

[1] 
https://lore.kernel.org/bpf/20261001102909.1152572-1-vineet.gupta@linux.dev/T/#t

>
>> -	int i;
>> -	int ret = 0;
>>   	int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
>>   	struct task_struct *g, *t;
>> +	int i, ret = 0;
>>
>>   	if (WARN_ON_ONCE(!fgraph_stack_cachep))
>>   		return -ENOMEM;
>> @@ -1054,26 +1053,37 @@ 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) {
>> +		for_each_process_thread(g, t) {
>> +			unsigned long *rs;
>> +
>> +			if (t->ret_stack)
>> +				continue;
>> +
>> +			rs = kmem_cache_alloc(fgraph_stack_cachep, GFP_NOWAIT);
>> +			if (!rs) {
>> +				/*
>> +				 * Returning from inside scoped_guard() drops
>> +				 * the RCU read lock, but skips the free loop
>> +				 * below. That is only safe because start ==
>> +				 * end here, which leaves that loop nothing to
>> +				 * free. Keep the two in step if this
>> +				 * exhaustion check ever changes.
>> +				 */
>> +				if (start == end)
>> +					return -EAGAIN;
>> +				rs = ret_stack_list[start++];
>> +			}
>>
>> -		if (t->ret_stack == NULL) {
>>   			atomic_set(&t->trace_overrun, 0);
>> -			ret_stack_init_task_vars(ret_stack_list[start]);
>> +			ret_stack_init_task_vars(rs);
>>   			t->curr_ret_stack = 0;
>>   			t->curr_ret_depth = -1;
>>   			/* Make sure the tasks see the 0 first: */
>> -			smp_wmb();
>> -			t->ret_stack = ret_stack_list[start++];
>> +			smp_store_release(&t->ret_stack, rs);
>>   		}
>>   	}
>>
>> -unlock:
>> -	rcu_read_unlock();
>>   free:
>>   	for (i = start; i < end; i++)
>>   		kmem_cache_free(fgraph_stack_cachep, ret_stack_list[i]);
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/36644771324