bpf: Avoid hashtab deadlock with map_locked

If a hashtab is accessed in both non-NMI and NMI context, the system may
deadlock on bucket->lock. Fix this issue with percpu counter map_locked.
map_locked rejects concurrent access to the same bucket from the same CPU.
To reduce memory overhead, map_locked is not added per bucket. Instead,
8 percpu counters are added to each hashtab. buckets are assigned to these
counters based on the lower bits of its hash.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201029071925.3103400-3-songliubraving@fb.com

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 278da031c91ab940283ce10a1b4e1143e210ea24..da59ba978d17230684276001dce1de8e7816cb12 100644 (file)
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -86,6 +86,9 @@ struct bucket {
        };
 };
 
+#define HASHTAB_MAP_LOCK_COUNT 8
+#define HASHTAB_MAP_LOCK_MASK (HASHTAB_MAP_LOCK_COUNT - 1)
+
 struct bpf_htab {
        struct bpf_map map;
        struct bucket *buckets;
@@ -100,6 +103,7 @@ 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 */
@@ -152,26 +156,41 @@ static void htab_init_buckets(struct bpf_htab *htab)
        }
 }
 
-static inline unsigned long htab_lock_bucket(const struct bpf_htab *htab,
-                                            struct bucket *b)
+static inline int htab_lock_bucket(const struct bpf_htab *htab,
+                                  struct bucket *b, u32 hash,
+                                  unsigned long *pflags)
 {
        unsigned long flags;
 
+       hash = hash & HASHTAB_MAP_LOCK_MASK;
+
+       migrate_disable();
+       if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
+               __this_cpu_dec(*(htab->map_locked[hash]));
+               migrate_enable();
+               return -EBUSY;
+       }
+
        if (htab_use_raw_lock(htab))
                raw_spin_lock_irqsave(&b->raw_lock, flags);
        else
                spin_lock_irqsave(&b->lock, flags);
-       return flags;
+       *pflags = flags;
+
+       return 0;
 }
 
 static inline void htab_unlock_bucket(const struct bpf_htab *htab,
-                                     struct bucket *b,
+                                     struct bucket *b, u32 hash,
                                      unsigned long flags)
 {
+       hash = hash & HASHTAB_MAP_LOCK_MASK;
        if (htab_use_raw_lock(htab))
                raw_spin_unlock_irqrestore(&b->raw_lock, flags);
        else
                spin_unlock_irqrestore(&b->lock, flags);
+       __this_cpu_dec(*(htab->map_locked[hash]));
+       migrate_enable();
 }
 
 static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node);
@@ -429,8 +448,8 @@ 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;
        u64 cost;
-       int err;
 
        htab = kzalloc(sizeof(*htab), GFP_USER);
        if (!htab)
@@ -487,6 +506,13 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
        if (!htab->buckets)
                goto free_charge;
 
+       for (i = 0; i < HASHTAB_MAP_LOCK_COUNT; i++) {
+               htab->map_locked[i] = __alloc_percpu_gfp(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
@@ -497,7 +523,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
        if (prealloc) {
                err = prealloc_init(htab);
                if (err)
-                       goto free_buckets;
+                       goto free_map_locked;
 
                if (!percpu && !lru) {
                        /* lru itself can remove the least used element, so
@@ -513,7 +539,9 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
 
 free_prealloc:
        prealloc_destroy(htab);
-free_buckets:
+free_map_locked:
+       for (i = 0; i < HASHTAB_MAP_LOCK_COUNT; i++)
+               free_percpu(htab->map_locked[i]);
        bpf_map_area_free(htab->buckets);
 free_charge:
        bpf_map_charge_finish(&htab->map.memory);
@@ -694,12 +722,15 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
        struct hlist_nulls_node *n;
        unsigned long flags;
        struct bucket *b;
+       int ret;
 
        tgt_l = container_of(node, struct htab_elem, lru_node);
        b = __select_bucket(htab, tgt_l->hash);
        head = &b->head;
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, tgt_l->hash, &flags);
+       if (ret)
+               return false;
 
        hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
                if (l == tgt_l) {
@@ -707,7 +738,7 @@ static bool htab_lru_map_delete_node(void *arg, struct bpf_lru_node *node)
                        break;
                }
 
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, tgt_l->hash, flags);
 
        return l == tgt_l;
 }
@@ -979,7 +1010,9 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
                 */
        }
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, hash, &flags);
+       if (ret)
+               return ret;
 
        l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1020,7 +1053,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
        }
        ret = 0;
 err:
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, hash, flags);
        return ret;
 }
 
@@ -1058,7 +1091,9 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
                return -ENOMEM;
        memcpy(l_new->key + round_up(map->key_size, 8), value, map->value_size);
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, hash, &flags);
+       if (ret)
+               return ret;
 
        l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1077,7 +1112,7 @@ static int htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value,
        ret = 0;
 
 err:
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, hash, flags);
 
        if (ret)
                bpf_lru_push_free(&htab->lru, &l_new->lru_node);
@@ -1112,7 +1147,9 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
        b = __select_bucket(htab, hash);
        head = &b->head;
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, hash, &flags);
+       if (ret)
+               return ret;
 
        l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1135,7 +1172,7 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
        }
        ret = 0;
 err:
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, hash, flags);
        return ret;
 }
 
@@ -1175,7 +1212,9 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
                        return -ENOMEM;
        }
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, hash, &flags);
+       if (ret)
+               return ret;
 
        l_old = lookup_elem_raw(head, hash, key, key_size);
 
@@ -1197,7 +1236,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
        }
        ret = 0;
 err:
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, hash, flags);
        if (l_new)
                bpf_lru_push_free(&htab->lru, &l_new->lru_node);
        return ret;
@@ -1225,7 +1264,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
        struct htab_elem *l;
        unsigned long flags;
        u32 hash, key_size;
-       int ret = -ENOENT;
+       int ret;
 
        WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
 
@@ -1235,17 +1274,20 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
        b = __select_bucket(htab, hash);
        head = &b->head;
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, hash, &flags);
+       if (ret)
+               return ret;
 
        l = lookup_elem_raw(head, hash, key, key_size);
 
        if (l) {
                hlist_nulls_del_rcu(&l->hash_node);
                free_htab_elem(htab, l);
-               ret = 0;
+       } else {
+               ret = -ENOENT;
        }
 
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, hash, flags);
        return ret;
 }
 
@@ -1257,7 +1299,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
        struct htab_elem *l;
        unsigned long flags;
        u32 hash, key_size;
-       int ret = -ENOENT;
+       int ret;
 
        WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
 
@@ -1267,16 +1309,18 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
        b = __select_bucket(htab, hash);
        head = &b->head;
 
-       flags = htab_lock_bucket(htab, b);
+       ret = htab_lock_bucket(htab, b, hash, &flags);
+       if (ret)
+               return ret;
 
        l = lookup_elem_raw(head, hash, key, key_size);
 
-       if (l) {
+       if (l)
                hlist_nulls_del_rcu(&l->hash_node);
-               ret = 0;
-       }
+       else
+               ret = -ENOENT;
 
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, hash, flags);
        if (l)
                bpf_lru_push_free(&htab->lru, &l->lru_node);
        return ret;
@@ -1302,6 +1346,7 @@ static void delete_all_elements(struct bpf_htab *htab)
 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.
@@ -1320,6 +1365,8 @@ static void htab_map_free(struct bpf_map *map)
        free_percpu(htab->extra_elems);
        bpf_map_area_free(htab->buckets);
        lockdep_unregister_key(&htab->lockdep_key);
+       for (i = 0; i < HASHTAB_MAP_LOCK_COUNT; i++)
+               free_percpu(htab->map_locked[i]);
        kfree(htab);
 }
 
@@ -1423,8 +1470,11 @@ again_nocopy:
        b = &htab->buckets[batch];
        head = &b->head;
        /* do not grab the lock unless need it (bucket_cnt > 0). */
-       if (locked)
-               flags = htab_lock_bucket(htab, b);
+       if (locked) {
+               ret = htab_lock_bucket(htab, b, batch, &flags);
+               if (ret)
+                       goto next_batch;
+       }
 
        bucket_cnt = 0;
        hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
@@ -1441,7 +1491,7 @@ again_nocopy:
                /* Note that since bucket_cnt > 0 here, it is implicit
                 * that the locked was grabbed, so release it.
                 */
-               htab_unlock_bucket(htab, b, flags);
+               htab_unlock_bucket(htab, b, batch, flags);
                rcu_read_unlock();
                bpf_enable_instrumentation();
                goto after_loop;
@@ -1452,7 +1502,7 @@ again_nocopy:
                /* Note that since bucket_cnt > 0 here, it is implicit
                 * that the locked was grabbed, so release it.
                 */
-               htab_unlock_bucket(htab, b, flags);
+               htab_unlock_bucket(htab, b, batch, flags);
                rcu_read_unlock();
                bpf_enable_instrumentation();
                kvfree(keys);
@@ -1505,7 +1555,7 @@ again_nocopy:
                dst_val += value_size;
        }
 
-       htab_unlock_bucket(htab, b, flags);
+       htab_unlock_bucket(htab, b, batch, flags);
        locked = false;
 
        while (node_to_free) {