netdev - Re: [PATCH v4 bpf-next 2/9] bpf: Add map side support for bpf timers.

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Date:   Thu, 1 Jul 2021 23:23:43 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     <davem@...emloft.net>, <daniel@...earbox.net>, <andrii@...nel.org>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 2/9] bpf: Add map side support for bpf timers.

On Thu, Jul 01, 2021 at 12:20:37PM -0700, Alexei Starovoitov wrote:
[ ... ]

> +static void htab_free_prealloced_timers(struct bpf_htab *htab)
> +{
> +	u32 num_entries = htab->map.max_entries;
> +	int i;
> +
> +	if (likely(!map_value_has_timer(&htab->map)))
> +		return;
> +	if (htab_has_extra_elems(htab))
> +		num_entries += num_possible_cpus();
> +
> +	for (i = 0; i < num_entries; i++) {
> +		struct htab_elem *elem;
> +
> +		elem = get_htab_elem(htab, i);
> +		bpf_timer_cancel_and_free(elem->key +
> +					  round_up(htab->map.key_size, 8) +
> +					  htab->map.timer_off);
> +		cond_resched();
> +	}
> +}
> +
[ ... ]

> +static void htab_free_malloced_timers(struct bpf_htab *htab)
> +{
> +	int i;
> +
> +	for (i = 0; i < htab->n_buckets; i++) {
> +		struct hlist_nulls_head *head = select_bucket(htab, i);
> +		struct hlist_nulls_node *n;
> +		struct htab_elem *l;
> +
> +		hlist_nulls_for_each_entry(l, n, head, hash_node)
It is called from map_release_uref() which is not under rcu.
Either a bucket lock or rcu_read_lock is needed here.

Another question, can prealloc map does the same thing
like here (i.e. walk the buckets) during map_release_uref()?