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

[bot+bpf-ci@kernel.org]

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> 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.

> -	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