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Message-ID: <fdb3b69c-a29c-2d5b-a122-9d98ea387fda@huawei.com>
Date: Thu, 24 Nov 2022 22:13:10 +0800
From: Hou Tao <houtao1@...wei.com>
To: Tonghao Zhang <xiangxia.m.yue@...il.com>
CC: <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
bpf <bpf@...r.kernel.org>
Subject: Re: [net-next] bpf: avoid hashtab deadlock with try_lock
Hi,
On 11/24/2022 8:57 PM, Tonghao Zhang wrote:
> On Tue, Nov 22, 2022 at 12:06 PM Hou Tao <houtao1@...wei.com> wrote:
>> Hi,
>>
>> On 11/22/2022 12:01 PM, Hou Tao wrote:
>>> Hi,
>>>
>>> On 11/22/2022 11:12 AM, Tonghao Zhang wrote:
>>>> .
>>>>
>>>> On Tue, Nov 22, 2022 at 9:16 AM Hou Tao <houtao1@...wei.com> wrote:
>>>>> Hi,
>>>>>
>>>>> On 11/21/2022 6:05 PM, xiangxia.m.yue@...il.com wrote:
>>>>>> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
>>>>>>
>>>>>> The commit 20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked"),
>>>>>> try to fix deadlock, but in some case, the deadlock occurs:
>>>>>>
>>>>>> * CPUn in task context with K1, and taking lock.
>>>>>> * CPUn interrupted by NMI context, with K2.
>>>>>> * They are using the same bucket, but different map_locked.
>>>>> It is possible when n_buckets is less than HASHTAB_MAP_LOCK_COUNT (e.g.,
>>>>> n_bucket=4). If using hash & min(HASHTAB_MAP_LOCK_MASK, n_bucket - 1) as the
>>>>> index of map_locked, I think the deadlock will be gone.
>>>> Yes, but for saving memory, HASHTAB_MAP_LOCK_MASK should not be too
>>>> large(now this value is 8-1).
>>>> if user define n_bucket ,e.g 8192, the part of bucket only are
>>>> selected via hash & min(HASHTAB_MAP_LOCK_MASK, n_bucket - 1).
>> I don't mean to extend map_locked. Using hash & min(HASHTAB_MAP_LOCK_MASK,
>> n_bucket - 1) as index of map_locked can guarantee the same map_locked will be
>> used if different update processes are using the same bucket lock.
> Thanks, I got it. but I tried it using the hash = hash &
> min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1) in
> htab_lock_bucket/htab_unlock_bucket.
> But the warning occur again.
Does the deadlock happen ? Or just get the warning from lockdep. Maybe it is
just a false alarm from lockdep. I will check tomorrow. Could you share the
steps on how to reproduce the problem, specially the size of the hash table ?
> > > SNIP
>>>>>> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
>>>>>> index 22855d6ff6d3..429acd97c869 100644
>>>>>> --- a/kernel/bpf/hashtab.c
>>>>>> +++ b/kernel/bpf/hashtab.c
>>>>>> @@ -80,9 +80,6 @@ struct bucket {
>>>>>> raw_spinlock_t raw_lock;
>>>>>> };
>>>>>>
>>>>>> -#define HASHTAB_MAP_LOCK_COUNT 8
>>>>>> -#define HASHTAB_MAP_LOCK_MASK (HASHTAB_MAP_LOCK_COUNT - 1)
>>>>>> -
>>>>>> struct bpf_htab {
>>>>>> struct bpf_map map;
>>>>>> struct bpf_mem_alloc ma;
>>>>>> @@ -104,7 +101,6 @@ struct bpf_htab {
>>>>>> u32 elem_size; /* size of each element in bytes */
>>>>>> u32 hashrnd;
>>>>>> struct lock_class_key lockdep_key;
>>>>>> - int __percpu *map_locked[HASHTAB_MAP_LOCK_COUNT];
>>>>>> };
>>>>>>
>>>>>> /* each htab element is struct htab_elem + key + value */
>>>>>> @@ -146,35 +142,26 @@ static void htab_init_buckets(struct bpf_htab *htab)
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> -static inline int htab_lock_bucket(const struct bpf_htab *htab,
>>>>>> - struct bucket *b, u32 hash,
>>>>>> +static inline int htab_lock_bucket(struct bucket *b,
>>>>>> unsigned long *pflags)
>>>>>> {
>>>>>> unsigned long flags;
>>>>>>
>>>>>> - hash = hash & HASHTAB_MAP_LOCK_MASK;
>>>>>> -
>>>>>> - preempt_disable();
>>>>>> - if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
>>>>>> - __this_cpu_dec(*(htab->map_locked[hash]));
>>>>>> - preempt_enable();
>>>>>> - return -EBUSY;
>>>>>> + if (in_nmi()) {
>>>>>> + if (!raw_spin_trylock_irqsave(&b->raw_lock, flags))
>>>>>> + return -EBUSY;
>>>>>> + } else {
>>>>>> + raw_spin_lock_irqsave(&b->raw_lock, flags);
>>>>>> }
>>>>>>
>>>>>> - raw_spin_lock_irqsave(&b->raw_lock, flags);
>>>>>> *pflags = flags;
>>>>>> -
>>>>>> return 0;
>>>>>> }
>>>>> map_locked is also used to prevent the re-entrance of htab_lock_bucket() on the
>>>>> same CPU, so only check in_nmi() is not enough.
>>>> NMI, IRQ, and preempt may interrupt the task context.
>>>> In htab_lock_bucket, raw_spin_lock_irqsave disable the preempt and
>>>> irq. so only NMI may interrupt the codes, right ?
>>> The re-entrance here means the nesting of bpf programs as show below:
>>>
>>> bpf_prog A
>>> update map X
>>> htab_lock_bucket
>>> raw_spin_lock_irqsave()
>>> lookup_elem_raw()
>>> // bpf prog B is attached on lookup_elem_raw()
> I am confused, bpf_prog A disables preempt and irq with
> raw_spin_lock_irqsave. Why bpf prog B run here?
Because program B (e.g., fentry program) has been attached on lookup_elem_raw(),
calling lookup_elem_raw() will call the fentry program first. I had written a
test case for the similar scenario in bpf selftests. The path of the test case
is tools/testing/selftests/bpf/prog_tests/htab_update.c, you can use ./test_prog
-t htab_update/reenter_update to run the test case.
>>> bpf prog B
>>> update map X again and update the element
>>> htab_lock_bucket()
>>> // dead-lock
>>> raw_spinlock_irqsave()
>>> .
>>>
>>>>>> -static inline void htab_unlock_bucket(const struct bpf_htab *htab,
>>>>>> - struct bucket *b, u32 hash,
>>>>>> +static inline void htab_unlock_bucket(struct bucket *b,
>>>>>> unsigned long flags)
>>>>>> {
>>>>>> - hash = hash & HASHTAB_MAP_LOCK_MASK;
>>>>>> raw_spin_unlock_irqrestore(&b->raw_lock, flags);
>>>>>> - __this_cpu_dec(*(htab->map_locked[hash]));
>>>>>> - preempt_enable();
>>>>>> }
>>>>>>
>>>>>> static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node);
>>>>>> @@ -467,7 +454,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>>>>>> bool percpu_lru = (attr->map_flags & BPF_F_NO_COMMON_LRU);
>>>>>> bool prealloc = !(attr->map_flags & BPF_F_NO_PREALLOC);
>>>>>> struct bpf_htab *htab;
>>>>>> - int err, i;
>>>>>> + int err;
>>>>>>
>>>>>> htab = bpf_map_area_alloc(sizeof(*htab), NUMA_NO_NODE);
>>>>>> if (!htab)
>>>>>> @@ -513,15 +500,6 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>>>>>> if (!htab->buckets)
>>>>>> goto free_htab;
>>>>>>
>>>>>> - for (i = 0; i < HASHTAB_MAP_LOCK_COUNT; i++) {
>>>>>> - htab->map_locked[i] = bpf_map_alloc_percpu(&htab->map,
>>>>>> - sizeof(int),
>>>>>> - sizeof(int),
>>>>>> - GFP_USER);
>>>>>> - if (!htab->map_locked[i])
>>>>>> - goto free_map_locked;
>>>>>> - }
>>>>>> -
>>>>>> if (htab->map.map_flags & BPF_F_ZERO_SEED)
>>>>>> htab->hashrnd = 0;
>>>>>> else
>>>>>> @@ -549,13 +527,13 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>>>>>> if (htab->use_percpu_counter) {
>>>>>> err = percpu_counter_init(&htab->pcount, 0, GFP_KERNEL);
>>>>>> if (err)
>>>>>> - goto free_map_locked;
>>>>>> + goto free_buckets;
>>>>>> }
>>>>>>
>>>>>> if (prealloc) {
>>>>>> err = prealloc_init(htab);
>>>>>> if (err)
>>>>>> - goto free_map_locked;
>>>>>> + goto free_buckets;
>>>>>>
>>>>>> if (!percpu && !lru) {
>>>>>> /* lru itself can remove the least used element, so
>>>>>> @@ -568,12 +546,12 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>>>>>> } else {
>>>>>> err = bpf_mem_alloc_init(&htab->ma, htab->elem_size, false);
>>>>>> if (err)
>>>>>> - goto free_map_locked;
>>>>>> + goto free_buckets;
>>>>>> if (percpu) {
>>>>>> err = bpf_mem_alloc_init(&htab->pcpu_ma,
>>>>>> round_up(htab->map.value_size, 8), true);
>>>>>> if (err)
>>>>>> - goto free_map_locked;
>>>>>> + goto free_buckets;
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> @@ -581,11 +559,10 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
>>>>>>
>>>>>> free_prealloc:
>>>>>> prealloc_destroy(htab);
>>>>>> -free_map_locked:
>>>>>> +free_buckets:
>>>>>> if (htab->use_percpu_counter)
>>>>>> percpu_counter_destroy(&htab->pcount);
>>>>>> - for (i = 0; i < HASHTAB_MAP_LOCK_COUNT; i++)
>>>>>> - free_percpu(htab->map_locked[i]);
>>>>>> +
>>>>>> bpf_map_area_free(htab->buckets);
>>>>>> bpf_mem_alloc_destroy(&htab->pcpu_ma);
>>>>>> bpf_mem_alloc_destroy(&htab->ma);
>>>>>> @@ -782,7 +759,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
>>>>>> b = __select_bucket(htab, tgt_l->hash);
>>>>>> head = &b->head;
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, tgt_l->hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return false;
>>>>>>
>>>>>> @@ -793,7 +770,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
>>>>>> break;
>>>>>> }
>>>>>>
>>>>>> - htab_unlock_bucket(htab, b, tgt_l->hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>>
>>>>>> return l == tgt_l;
>>>>>> }
>>>>>> @@ -1107,7 +1084,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
>>>>>> */
>>>>>> }
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1152,7 +1129,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
>>>>>> }
>>>>>> ret = 0;
>>>>>> err:
>>>>>> - htab_unlock_bucket(htab, b, hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> return ret;
>>>>>> }
>>>>>>
>>>>>> @@ -1198,7 +1175,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
>>>>>> copy_map_value(&htab->map,
>>>>>> l_new->key + round_up(map->key_size, 8), value);
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1219,7 +1196,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
>>>>>> ret = 0;
>>>>>>
>>>>>> err:
>>>>>> - htab_unlock_bucket(htab, b, hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>>
>>>>>> if (ret)
>>>>>> htab_lru_push_free(htab, l_new);
>>>>>> @@ -1255,7 +1232,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
>>>>>> b = __select_bucket(htab, hash);
>>>>>> head = &b->head;
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1280,7 +1257,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
>>>>>> }
>>>>>> ret = 0;
>>>>>> err:
>>>>>> - htab_unlock_bucket(htab, b, hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> return ret;
>>>>>> }
>>>>>>
>>>>>> @@ -1321,7 +1298,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
>>>>>> return -ENOMEM;
>>>>>> }
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1345,7 +1322,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
>>>>>> }
>>>>>> ret = 0;
>>>>>> err:
>>>>>> - htab_unlock_bucket(htab, b, hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> if (l_new)
>>>>>> bpf_lru_push_free(&htab->lru, &l_new->lru_node);
>>>>>> return ret;
>>>>>> @@ -1384,7 +1361,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
>>>>>> b = __select_bucket(htab, hash);
>>>>>> head = &b->head;
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1397,7 +1374,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
>>>>>> ret = -ENOENT;
>>>>>> }
>>>>>>
>>>>>> - htab_unlock_bucket(htab, b, hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> return ret;
>>>>>> }
>>>>>>
>>>>>> @@ -1420,7 +1397,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
>>>>>> b = __select_bucket(htab, hash);
>>>>>> head = &b->head;
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1431,7 +1408,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
>>>>>> else
>>>>>> ret = -ENOENT;
>>>>>>
>>>>>> - htab_unlock_bucket(htab, b, hash, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> if (l)
>>>>>> htab_lru_push_free(htab, l);
>>>>>> return ret;
>>>>>> @@ -1494,7 +1471,6 @@ static void htab_map_free_timers(struct bpf_map *map)
>>>>>> static void htab_map_free(struct bpf_map *map)
>>>>>> {
>>>>>> struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
>>>>>> - int i;
>>>>>>
>>>>>> /* bpf_free_used_maps() or close(map_fd) will trigger this map_free callback.
>>>>>> * bpf_free_used_maps() is called after bpf prog is no longer executing.
>>>>>> @@ -1517,10 +1493,10 @@ static void htab_map_free(struct bpf_map *map)
>>>>>> bpf_map_area_free(htab->buckets);
>>>>>> bpf_mem_alloc_destroy(&htab->pcpu_ma);
>>>>>> bpf_mem_alloc_destroy(&htab->ma);
>>>>>> +
>>>>>> if (htab->use_percpu_counter)
>>>>>> percpu_counter_destroy(&htab->pcount);
>>>>>> - for (i = 0; i < HASHTAB_MAP_LOCK_COUNT; i++)
>>>>>> - free_percpu(htab->map_locked[i]);
>>>>>> +
>>>>>> lockdep_unregister_key(&htab->lockdep_key);
>>>>>> bpf_map_area_free(htab);
>>>>>> }
>>>>>> @@ -1564,7 +1540,7 @@ static int __htab_map_lookup_and_delete_elem(struct bpf_map *map, void *key,
>>>>>> b = __select_bucket(htab, hash);
>>>>>> head = &b->head;
>>>>>>
>>>>>> - ret = htab_lock_bucket(htab, b, hash, &bflags);
>>>>>> + ret = htab_lock_bucket(b, &bflags);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> @@ -1602,7 +1578,7 @@ static int __htab_map_lookup_and_delete_elem(struct bpf_map *map, void *key,
>>>>>> free_htab_elem(htab, l);
>>>>>> }
>>>>>>
>>>>>> - htab_unlock_bucket(htab, b, hash, bflags);
>>>>>> + htab_unlock_bucket(b, bflags);
>>>>>>
>>>>>> if (is_lru_map && l)
>>>>>> htab_lru_push_free(htab, l);
>>>>>> @@ -1720,7 +1696,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>>>>>> head = &b->head;
>>>>>> /* do not grab the lock unless need it (bucket_cnt > 0). */
>>>>>> if (locked) {
>>>>>> - ret = htab_lock_bucket(htab, b, batch, &flags);
>>>>>> + ret = htab_lock_bucket(b, &flags);
>>>>>> if (ret) {
>>>>>> rcu_read_unlock();
>>>>>> bpf_enable_instrumentation();
>>>>>> @@ -1743,7 +1719,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>>>>>> /* Note that since bucket_cnt > 0 here, it is implicit
>>>>>> * that the locked was grabbed, so release it.
>>>>>> */
>>>>>> - htab_unlock_bucket(htab, b, batch, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> rcu_read_unlock();
>>>>>> bpf_enable_instrumentation();
>>>>>> goto after_loop;
>>>>>> @@ -1754,7 +1730,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>>>>>> /* Note that since bucket_cnt > 0 here, it is implicit
>>>>>> * that the locked was grabbed, so release it.
>>>>>> */
>>>>>> - htab_unlock_bucket(htab, b, batch, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> rcu_read_unlock();
>>>>>> bpf_enable_instrumentation();
>>>>>> kvfree(keys);
>>>>>> @@ -1815,7 +1791,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>>>>>> dst_val += value_size;
>>>>>> }
>>>>>>
>>>>>> - htab_unlock_bucket(htab, b, batch, flags);
>>>>>> + htab_unlock_bucket(b, flags);
>>>>>> locked = false;
>>>>>>
>>>>>> while (node_to_free) {
>>> .
>
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