Re: [PATCHv5 bpf-next 4/9] ftrace: Add update_ftrace_direct_add function

Steven Rostedt
2025-12-18T01:39:13Z

On Mon, 15 Dec 2025 22:13:57 +0100
Jiri Olsa <jolsa@kernel.org> wrote:

> +/**
> + * hash_add - adds two struct ftrace_hash and returns the result
> + * @a: struct ftrace_hash object
> + * @b: struct ftrace_hash object
> + *
> + * Returns struct ftrace_hash object on success, NULL on error.
> + */
> +static struct ftrace_hash *hash_add(struct ftrace_hash *a, struct ftrace_hash *b)
> +{
> + struct ftrace_func_entry *entry;
> + struct ftrace_hash *add;
> + int size, i;
> +
> + size = hash_count(a) + hash_count(b);
> + if (size > 32)
> + size = 32;
> +
> + add = alloc_and_copy_ftrace_hash(fls(size), a);
> + if (!add)
> + goto error;

You can just return NULL here, as add is NULL.

> +
> + size = 1 << b->size_bits;
> + for (i = 0; i < size; i++) {
> + hlist_for_each_entry(entry, &b->buckets[i], hlist) {
> + if (add_hash_entry_direct(add, entry->ip, entry->direct) == NULL)
> + goto error;

Could remove the error and have:

if (add_hash_entry_direct(add, entry->ip, entry->direct) == NULL) {
free_ftrace_hash(add);
return NULL;
}

> + }
> + }
> + return add;
> +
> + error:
> + free_ftrace_hash(add);
> + return NULL;
> +}
> +

Non static functions require a kerneldoc header.

> +int update_ftrace_direct_add(struct ftrace_ops *ops, struct ftrace_hash *hash)
> +{
> + struct ftrace_hash *old_direct_functions = NULL, *new_direct_functions;
> + struct ftrace_hash *old_filter_hash, *new_filter_hash = NULL;

BTW, I prefer to not double up on variables. That is to have each on
their own lines. Makes it easier to read for me.

> + struct ftrace_func_entry *entry;
> + int i, size, err = -EINVAL;

Even here.

> + bool reg;
> +
> + if (!hash_count(hash))
> + return -EINVAL;
> +
> + mutex_lock(&direct_mutex);
> +
> + /* Make sure requested entries are not already registered. */
> + size = 1 << hash->size_bits;
> + for (i = 0; i < size; i++) {

If you want, you can remove the i declaration and use for(int i = 0; ... here.

> + hlist_for_each_entry(entry, &hash->buckets[i], hlist) {
> + if (__ftrace_lookup_ip(direct_functions, entry->ip))
> + goto out_unlock;
> + }
> + }
> +
> + old_filter_hash = ops->func_hash ? ops->func_hash->filter_hash : NULL;
> +
> + /* If there's nothing in filter_hash we need to register the ops. */
> + reg = hash_count(old_filter_hash) == 0;
> + if (reg) {
> + if (ops->func || ops->trampoline)
> + goto out_unlock;
> + if (ops->flags & FTRACE_OPS_FL_ENABLED)
> + goto out_unlock;
> + }
> +
> + err = -ENOMEM;
> + new_filter_hash = hash_add(old_filter_hash, hash);
> + if (!new_filter_hash)
> + goto out_unlock;
> +
> + new_direct_functions = hash_add(direct_functions, hash);
> + if (!new_direct_functions)
> + goto out_unlock;
> +
> + old_direct_functions = direct_functions;
> + rcu_assign_pointer(direct_functions, new_direct_functions);
> +
> + if (reg) {
> + ops->func = call_direct_funcs;
> + ops->flags |= MULTI_FLAGS;
> + ops->trampoline = FTRACE_REGS_ADDR;
> + ops->local_hash.filter_hash = new_filter_hash;
> +
> + err = register_ftrace_function_nolock(ops);
> + if (err) {
> + /* restore old filter on error */
> + ops->local_hash.filter_hash = old_filter_hash;
> +
> + /* cleanup for possible another register call */
> + ops->func = NULL;
> + ops->trampoline = 0;
> + } else {
> + new_filter_hash = old_filter_hash;
> + }
> + } else {
> + err = ftrace_update_ops(ops, new_filter_hash, EMPTY_HASH);
> + /*
> + * new_filter_hash is dup-ed, so we need to release it anyway,
> + * old_filter_hash either stays on error or is already released
> + */
> + }
> +
> + if (err) {
> + /* reset direct_functions and free the new one */
> + rcu_assign_pointer(direct_functions, old_direct_functions);
> + old_direct_functions = new_direct_functions;
> + }
> +
> + out_unlock:
> + mutex_unlock(&direct_mutex);
> +
> + if (old_direct_functions && old_direct_functions != EMPTY_HASH)
> + call_rcu_tasks(&old_direct_functions->rcu, register_ftrace_direct_cb);
> + if (new_filter_hash)

free_ftrace_hash() checks for NULL, so you don't need the above if statement.

> + free_ftrace_hash(new_filter_hash);
> +
> + return err;

-- Steve

> +}
> +
> #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
>
> /**