From: Kui-Feng Lee <sinquersw@gmail.com>
To: Stanislav Fomichev <sdf@google.com>, Kui-Feng Lee <kuifeng@meta.com>
Cc: bpf@vger.kernel.org, ast@kernel.org, martin.lau@linux.dev,
song@kernel.org, kernel-team@meta.com, andrii@kernel.org
Subject: Re: [PATCH bpf-next v3 1/8] bpf: Maintain the refcount of struct_ops maps directly.
Date: Mon, 6 Mar 2023 13:45:25 -0800 [thread overview]
Message-ID: <6b1dbf32-67ff-027e-0d57-c3273339dc97@gmail.com> (raw)
In-Reply-To: <ZAZIn9DrvvYh5/QL@google.com>
On 3/6/23 12:10, Stanislav Fomichev wrote:
> On 03/02, Kui-Feng Lee wrote:
>> The refcount of the kvalue for struct_ops was quite intricate to keep
>> track of. By no longer utilizing it and replacing it with the refcount
>> from the struct_ops map, this process became more transparent and
>> uncomplicated.
>
>> Signed-off-by: Kui-Feng Lee <kuifeng@meta.com>
>> ---
>> include/linux/bpf.h | 3 ++
>> kernel/bpf/bpf_struct_ops.c | 84 ++++++++++++++++++++++---------------
>> kernel/bpf/syscall.c | 22 ++++++----
>> 3 files changed, 68 insertions(+), 41 deletions(-)
>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 8b5d0b4c4ada..cb837f42b99d 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -78,6 +78,7 @@ struct bpf_map_ops {
>> struct bpf_map *(*map_alloc)(union bpf_attr *attr);
>> void (*map_release)(struct bpf_map *map, struct file *map_file);
>> void (*map_free)(struct bpf_map *map);
>> + void (*map_free_rcu)(struct bpf_map *map);
>> int (*map_get_next_key)(struct bpf_map *map, void *key, void
>> *next_key);
>> void (*map_release_uref)(struct bpf_map *map);
>> void *(*map_lookup_elem_sys_only)(struct bpf_map *map, void *key);
>> @@ -1869,8 +1870,10 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd);
>> struct bpf_map *__bpf_map_get(struct fd f);
>> void bpf_map_inc(struct bpf_map *map);
>> void bpf_map_inc_with_uref(struct bpf_map *map);
>> +struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref);
>> struct bpf_map * __must_check bpf_map_inc_not_zero(struct bpf_map
>> *map);
>> void bpf_map_put_with_uref(struct bpf_map *map);
>> +void bpf_map_free_deferred(struct work_struct *work);
>> void bpf_map_put(struct bpf_map *map);
>> void *bpf_map_area_alloc(u64 size, int numa_node);
>> void *bpf_map_area_mmapable_alloc(u64 size, int numa_node);
>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>> index ece9870cab68..bba03b6b010b 100644
>> --- a/kernel/bpf/bpf_struct_ops.c
>> +++ b/kernel/bpf/bpf_struct_ops.c
>> @@ -58,6 +58,8 @@ struct bpf_struct_ops_map {
>> struct bpf_struct_ops_value kvalue;
>> };
>
>> +static DEFINE_MUTEX(update_mutex);
>
> Defined but unused?
>
>> +
>> #define VALUE_PREFIX "bpf_struct_ops_"
>> #define VALUE_PREFIX_LEN (sizeof(VALUE_PREFIX) - 1)
>
>> @@ -249,6 +251,7 @@ int bpf_struct_ops_map_sys_lookup_elem(struct
>> bpf_map *map, void *key,
>> struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map
>> *)map;
>> struct bpf_struct_ops_value *uvalue, *kvalue;
>> enum bpf_struct_ops_state state;
>> + s64 refcnt;
>
>> if (unlikely(*(u32 *)key != 0))
>> return -ENOENT;
>> @@ -261,13 +264,13 @@ int bpf_struct_ops_map_sys_lookup_elem(struct
>> bpf_map *map, void *key,
>> return 0;
>> }
>
>> - /* No lock is needed. state and refcnt do not need
>> - * to be updated together under atomic context.
>> - */
>> uvalue = value;
>> memcpy(uvalue, st_map->uvalue, map->value_size);
>> uvalue->state = state;
>> - refcount_set(&uvalue->refcnt, refcount_read(&kvalue->refcnt));
>> +
>> + refcnt = atomic64_read(&map->refcnt) - atomic64_read(&map->usercnt);
>> + refcount_set(&uvalue->refcnt,
>> + refcnt > 0 ? refcnt : 0);
>
>> return 0;
>> }
>> @@ -491,7 +494,6 @@ static int bpf_struct_ops_map_update_elem(struct
>> bpf_map *map, void *key,
>> *(unsigned long *)(udata + moff) = prog->aux->id;
>> }
>
>> - refcount_set(&kvalue->refcnt, 1);
>> bpf_map_inc(map);
>
>> set_memory_rox((long)st_map->image, 1);
>> @@ -536,8 +538,7 @@ static int bpf_struct_ops_map_delete_elem(struct
>> bpf_map *map, void *key)
>> switch (prev_state) {
>> case BPF_STRUCT_OPS_STATE_INUSE:
>> st_map->st_ops->unreg(&st_map->kvalue.data);
>> - if (refcount_dec_and_test(&st_map->kvalue.refcnt))
>> - bpf_map_put(map);
>> + bpf_map_put(map);
>> return 0;
>> case BPF_STRUCT_OPS_STATE_TOBEFREE:
>> return -EINPROGRESS;
>> @@ -582,6 +583,38 @@ static void bpf_struct_ops_map_free(struct
>> bpf_map *map)
>> bpf_map_area_free(st_map);
>> }
>
>> +static void bpf_struct_ops_map_free_wq(struct rcu_head *head)
>> +{
>> + struct bpf_struct_ops_map *st_map;
>> +
>> + st_map = container_of(head, struct bpf_struct_ops_map, rcu);
>> +
>> + /* bpf_map_free_deferred should not be called in a RCU callback. */
>> + INIT_WORK(&st_map->map.work, bpf_map_free_deferred);
>> + queue_work(system_unbound_wq, &st_map->map.work);
>> +}
>> +
>> +static void bpf_struct_ops_map_free_rcu(struct bpf_map *map)
>> +{
>> + struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map
>> *)map;
>> +
>> + /* Wait for a grace period of RCU. Then, post the map_free
>> + * work to the system_unbound_wq workqueue to free resources.
>> + *
>> + * The struct_ops's function may switch to another struct_ops.
>> + *
>> + * For example, bpf_tcp_cc_x->init() may switch to
>> + * another tcp_cc_y by calling
>> + * setsockopt(TCP_CONGESTION, "tcp_cc_y").
>> + * During the switch, bpf_struct_ops_put(tcp_cc_x) is called
>> + * and its refcount may reach 0 which then free its
>> + * trampoline image while tcp_cc_x is still running.
>
> "is still running" where? Why existing deferred work doesn't protect
> against this condition?
bpf_module_put() is protected by rcu_read_lock() alone. As the comment
mentions, setsockopt() can lead to a switch to another algorithm. It is
exposed to the BPF world as bpf_setscokopt(), so it could potentially
call bpf_struct_ops_put() while a BPF program is running. In order to
guarantee we don't free the trampoline while it is still running, we
have to wait for a grace period.
>
>> + *
>> + * Thus, a rcu grace period is needed here.
>> + */
>> + call_rcu(&st_map->rcu, bpf_struct_ops_map_free_wq);
>> +}
>> +
>> static int bpf_struct_ops_map_alloc_check(union bpf_attr *attr)
>> {
>> if (attr->key_size != sizeof(unsigned int) || attr->max_entries
>> != 1 ||
>> @@ -646,6 +679,7 @@ const struct bpf_map_ops bpf_struct_ops_map_ops = {
>> .map_alloc_check = bpf_struct_ops_map_alloc_check,
>> .map_alloc = bpf_struct_ops_map_alloc,
>
> [..]
>
>> .map_free = bpf_struct_ops_map_free,
>
> Since we have map_free_rcu check in bpf_map_put, does it mean the above
> is not needed?
It will be called by bpf_map_free_deferred().
>
>> + .map_free_rcu = bpf_struct_ops_map_free_rcu,
>> .map_get_next_key = bpf_struct_ops_map_get_next_key,
>> .map_lookup_elem = bpf_struct_ops_map_lookup_elem,
>> .map_delete_elem = bpf_struct_ops_map_delete_elem,
>> @@ -660,41 +694,23 @@ const struct bpf_map_ops bpf_struct_ops_map_ops = {
>> bool bpf_struct_ops_get(const void *kdata)
>> {
>> struct bpf_struct_ops_value *kvalue;
>> + struct bpf_struct_ops_map *st_map;
>> + struct bpf_map *map;
>
>> kvalue = container_of(kdata, struct bpf_struct_ops_value, data);
>> + st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
>
>> - return refcount_inc_not_zero(&kvalue->refcnt);
>> -}
>> -
>> -static void bpf_struct_ops_put_rcu(struct rcu_head *head)
>> -{
>> - struct bpf_struct_ops_map *st_map;
>> -
>> - st_map = container_of(head, struct bpf_struct_ops_map, rcu);
>> - bpf_map_put(&st_map->map);
>> + map = __bpf_map_inc_not_zero(&st_map->map, false);
>> + return !IS_ERR(map);
>> }
>
>> void bpf_struct_ops_put(const void *kdata)
>> {
>> struct bpf_struct_ops_value *kvalue;
>> + struct bpf_struct_ops_map *st_map;
>
>> kvalue = container_of(kdata, struct bpf_struct_ops_value, data);
>> - if (refcount_dec_and_test(&kvalue->refcnt)) {
>> - struct bpf_struct_ops_map *st_map;
>> -
>> - st_map = container_of(kvalue, struct bpf_struct_ops_map,
>> - kvalue);
>> - /* The struct_ops's function may switch to another struct_ops.
>> - *
>> - * For example, bpf_tcp_cc_x->init() may switch to
>> - * another tcp_cc_y by calling
>> - * setsockopt(TCP_CONGESTION, "tcp_cc_y").
>> - * During the switch, bpf_struct_ops_put(tcp_cc_x) is called
>> - * and its map->refcnt may reach 0 which then free its
>> - * trampoline image while tcp_cc_x is still running.
>> - *
>> - * Thus, a rcu grace period is needed here.
>> - */
>> - call_rcu(&st_map->rcu, bpf_struct_ops_put_rcu);
>> - }
>> + st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
>> +
>> + bpf_map_put(&st_map->map);
>> }
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index cda8d00f3762..358a0e40555e 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -684,7 +684,7 @@ void bpf_obj_free_fields(const struct btf_record
>> *rec, void *obj)
>> }
>
>> /* called from workqueue */
>> -static void bpf_map_free_deferred(struct work_struct *work)
>> +void bpf_map_free_deferred(struct work_struct *work)
>> {
>> struct bpf_map *map = container_of(work, struct bpf_map, work);
>> struct btf_field_offs *foffs = map->field_offs;
>> @@ -715,6 +715,15 @@ static void bpf_map_put_uref(struct bpf_map *map)
>> }
>> }
>
>> +static void bpf_map_put_wq(struct bpf_map *map)
>> +{
>> + INIT_WORK(&map->work, bpf_map_free_deferred);
>> + /* Avoid spawning kworkers, since they all might contend
>> + * for the same mutex like slab_mutex.
>> + */
>> + queue_work(system_unbound_wq, &map->work);
>> +}
>> +
>> /* decrement map refcnt and schedule it for freeing via workqueue
>> * (underlying map implementation ops->map_free() might sleep)
>> */
>> @@ -724,11 +733,10 @@ void bpf_map_put(struct bpf_map *map)
>> /* bpf_map_free_id() must be called first */
>> bpf_map_free_id(map);
>> btf_put(map->btf);
>> - INIT_WORK(&map->work, bpf_map_free_deferred);
>> - /* Avoid spawning kworkers, since they all might contend
>> - * for the same mutex like slab_mutex.
>> - */
>> - queue_work(system_unbound_wq, &map->work);
>> + if (map->ops->map_free_rcu)
>> + map->ops->map_free_rcu(map);
>> + else
>> + bpf_map_put_wq(map);
>> }
>> }
>> EXPORT_SYMBOL_GPL(bpf_map_put);
>> @@ -1276,7 +1284,7 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd)
>> }
>
>> /* map_idr_lock should have been held */
>> -static struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map,
>> bool uref)
>> +struct bpf_map *__bpf_map_inc_not_zero(struct bpf_map *map, bool uref)
>> {
>> int refold;
>
>> --
>> 2.30.2
>
next prev parent reply other threads:[~2023-03-06 21:45 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-03 1:21 [PATCH bpf-next v3 0/8] Transit between BPF TCP congestion controls Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 1/8] bpf: Maintain the refcount of struct_ops maps directly Kui-Feng Lee
2023-03-06 20:10 ` Stanislav Fomichev
2023-03-06 21:45 ` Kui-Feng Lee [this message]
2023-03-06 23:16 ` Martin KaFai Lau
2023-03-06 23:54 ` Kui-Feng Lee
2023-03-07 0:36 ` Martin KaFai Lau
2023-03-03 1:21 ` [PATCH bpf-next v3 2/8] bpf: Create links for BPF struct_ops maps Kui-Feng Lee
2023-03-06 20:23 ` Stanislav Fomichev
2023-03-06 22:02 ` Kui-Feng Lee
2023-03-07 2:11 ` Martin KaFai Lau
2023-03-07 18:04 ` Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 3/8] net: Update an existing TCP congestion control algorithm Kui-Feng Lee
2023-03-07 2:17 ` Martin KaFai Lau
2023-03-07 19:17 ` Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 4/8] libbpf: Create a bpf_link in bpf_map__attach_struct_ops() Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 5/8] bpf: Update the struct_ops of a bpf_link Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 6/8] libbpf: Update a bpf_link with another struct_ops Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 7/8] libbpf: Use .struct_ops.link section to indicate a struct_ops with a link Kui-Feng Lee
2023-03-03 1:21 ` [PATCH bpf-next v3 8/8] selftests/bpf: Test switching TCP Congestion Control algorithms Kui-Feng Lee
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