Re: [PATCH bpf-next v5 1/8] bpf: Retire the struct_ops map kvalue->refcnt.

Kui-Feng Lee
2023-03-08T22:30:56Z

On 3/8/23 10:30, Martin KaFai Lau wrote:
> On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 6792a7940e1e..50cfc2388cbc 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);
>
> This is no longer needed...
>
>>       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);
>> @@ -1934,6 +1935,7 @@ 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_put(struct bpf_map *map);
>> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
>> index 38903fb52f98..9e097fcc9cf4 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);
>
> Please address or reply to the earlier review comments. Stan had
> mentioned in v3 that this mutex is unused in this patch.
>
> This is only used in patch 5 of this set. Please move it there.

I will address this.

>
>> +
>>   #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;
>> @@ -267,7 +270,9 @@ int bpf_struct_ops_map_sys_lookup_elem(struct
>> bpf_map *map, void *key,
>>       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, max_t(s64, refcnt, 0));
>
> Please explain a few words that why it will work good enough and no need
> to be very accurate (eg. it is for introspection purpose to give an idea
> on how many refcnts are held by a subsystem, eg tcp_sock ...). This was
> also a comment given in v3.

Sure

>
>>       return 0;
>>   }
>> @@ -491,7 +496,6 @@ 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 +540,7 @@ 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;
>> @@ -574,6 +577,19 @@ static void bpf_struct_ops_map_free(struct
>> bpf_map *map)
>>   {
>>       struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map
>> *)map;
>> +    /* 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.
>> +     *
>> +     * Thus, a rcu grace period is needed here.
>> +     */
>> +    synchronize_rcu();
>> +
>>       if (st_map->links)
>>           bpf_struct_ops_map_put_progs(st_map);
>>       bpf_map_area_free(st_map->links);
>> @@ -676,41 +692,23 @@ 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 f406dfa13792..03273cddd6bd 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -1288,7 +1288,7 @@ struct bpf_map *bpf_map_get_with_uref(u32 ufd)
>>   }
>>   /* map_idr_lock should have been held */
>
> This comment needs to change, eg.
> map_idr_lock should have been held or the map is protected by rcu gp....
>

Sure.

>> -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;
>