内核hlist的使用

189 struct hlist_head {
190         struct hlist_node *first;
191 };
192 
193 struct hlist_node {
194         struct hlist_node *next, **pprev;
195 };

hlist_head表示哈希表的头结点，是hash数组的一个元素。

哈希表中每一个entry(hlist_head)所对应的都是一个链表（hlist)，该链表的结点由hlist_node表示。

hlist_head结构体只有一个域，即first。 first指针指向该hlist链表的第一个节点。

hlist_node结构体有两个域，next 和pprev。 next指针很容易理解，它指向下个hlist_node结点，倘若该节点是链表的最后一个节点，next指向NULL。

pprev是一个二级指针， 它指向前一个节点的next指针。为什么我们需要这样一个指针呢？它的好处是什么？

在回答这个问题之前，我们先研究另一个问题：为什么散列表的实现需要两个不同的数据结构？

散列表的目的是为了方便快速的查找，所以散列表通常是一个比较大的数组，否则“冲突”的概率会非常大， 这样也就失去了散列表的意义。如何做到既能维护一张大表，又能不使用过多的内存呢？就只能从数据结构上下功夫了。所以对于散列表的每个entry，它的结构体中只存放一个指针，解决了占用空间的问题。

现在又出现了另一个问题：数据结构不一致。显然，如果hlist_node采用传统的next,prev指针， 对于第一个节点和后面其他节点的处理会不一致。这样并不优雅，而且效率上也有损失。

hlist_node巧妙地将pprev指向上一个节点的next指针的地址，由于hlist_head和hlist_node指向的下一个节点的指针类型相同，这样就解决了通用性！

 31 /**
 32  * struct inet_frag_queue - fragment queue
 33  *
 34  * @lock: spinlock protecting the queue
 35  * @timer: queue expiration timer
 36  * @list: hash bucket list
 37  * @refcnt: reference count of the queue
 38  * @fragments: received fragments head
 39  * @fragments_tail: received fragments tail
 40  * @stamp: timestamp of the last received fragment
 41  * @len: total length of the original datagram
 42  * @meat: length of received fragments so far
 43  * @flags: fragment queue flags
 44  * @max_size: maximum received fragment size
 45  * @net: namespace that this frag belongs to
 46  * @list_evictor: list of queues to forcefully evict (e.g. due to low memory)
 47  */
 48 struct inet_frag_queue {
 49         spinlock_t              lock;
 50         struct timer_list       timer;
 51         struct hlist_node       list;   //hash节点
 52         atomic_t                refcnt;
 53         struct sk_buff          *fragments;
 54         struct sk_buff          *fragments_tail;
 55         ktime_t                 stamp;
 56         int                     len;
 57         int                     meat;
 58         __u8                    flags;
 59         u16                     max_size;
 60         struct netns_frags      *net;
 61         struct hlist_node       list_evictor;
 62 };
 63 
 64 #define INETFRAGS_HASHSZ        1024  //hash数组的大小
 65 
 66 /* averaged:
 67  * max_depth = default ipfrag_high_thresh / INETFRAGS_HASHSZ /
 68  *             rounded up (SKB_TRUELEN(0) + sizeof(struct ipq or
 69  *             struct frag_queue))
 70  */
 71 #define INETFRAGS_MAXDEPTH      128
 72 
 73 struct inet_frag_bucket {
 74         struct hlist_head       chain;  //hash冲突链表
 75         spinlock_t              chain_lock;
 76 };
 77 
 78 struct inet_frags {
 79         struct inet_frag_bucket hash[INETFRAGS_HASHSZ];  //hash数组
 80 
 81         struct work_struct      frags_work;
 82         unsigned int next_bucket;
 83         unsigned long last_rebuild_jiffies;
 84         bool rebuild;
 85 
 86         /* The first call to hashfn is responsible to initialize
 87          * rnd. This is best done with net_get_random_once.
 88          *
 89          * rnd_seqlock is used to let hash insertion detect
 90          * when it needs to re-lookup the hash chain to use.
 91          */
 92         u32                     rnd;
 93         seqlock_t               rnd_seqlock;
 94         int                     qsize;
 95 
 96         unsigned int            (*hashfn)(const struct inet_frag_queue *);
 97         bool                    (*match)(const struct inet_frag_queue *q,
 98                                          const void *arg);
 99         void                    (*constructor)(struct inet_frag_queue *q,
100                                                const void *arg);
101         void                    (*destructor)(struct inet_frag_queue *);
102         void                    (*skb_free)(struct sk_buff *);
103         void                    (*frag_expire)(unsigned long data);
104         struct kmem_cache       *frags_cachep;
105         const char              *frags_cache_name;
106 };

  1 /*
  2  * inet fragments management
  3  *
  4  *              This program is free software; you can redistribute it and/or
  5  *              modify it under the terms of the GNU General Public License
  6  *              as published by the Free Software Foundation; either version
  7  *              2 of the License, or (at your option) any later version.
  8  *
  9  *              Authors:        Pavel Emelyanov <xemul@openvz.org>
 10  *                              Started as consolidation of ipv4/ip_fragment.c,
 11  *                              ipv6/reassembly. and ipv6 nf conntrack reassembly
 12  */
 13 
 14 #include <linux/list.h>
 15 #include <linux/spinlock.h>
 16 #include <linux/module.h>
 17 #include <linux/timer.h>
 18 #include <linux/mm.h>
 19 #include <linux/random.h>
 20 #include <linux/skbuff.h>
 21 #include <linux/rtnetlink.h>
 22 #include <linux/slab.h>
 23 
 24 #include <net/sock.h>
 25 #include <net/inet_frag.h>
 26 #include <net/inet_ecn.h>
 27 
 28 #define INETFRAGS_EVICT_BUCKETS   128
 29 #define INETFRAGS_EVICT_MAX       512
 30 
 31 /* don‘t rebuild inetfrag table with new secret more often than this */
 32 #define INETFRAGS_MIN_REBUILD_INTERVAL (5 * HZ)
 33 
 34 /* Given the OR values of all fragments, apply RFC 3168 5.3 requirements
 35  * Value : 0xff if frame should be dropped.
 36  *         0 or INET_ECN_CE value, to be ORed in to final iph->tos field
 37  */
 38 const u8 ip_frag_ecn_table[16] = {
 39         /* at least one fragment had CE, and others ECT_0 or ECT_1 */
 40         [IPFRAG_ECN_CE | IPFRAG_ECN_ECT_0]                      = INET_ECN_CE,
 41         [IPFRAG_ECN_CE | IPFRAG_ECN_ECT_1]                      = INET_ECN_CE,
 42         [IPFRAG_ECN_CE | IPFRAG_ECN_ECT_0 | IPFRAG_ECN_ECT_1]   = INET_ECN_CE,
 43 
 44         /* invalid combinations : drop frame */
 45         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_CE] = 0xff,
 46         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_ECT_0] = 0xff,
 47         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_ECT_1] = 0xff,
 48         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_ECT_0 | IPFRAG_ECN_ECT_1] = 0xff,
 49         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_CE | IPFRAG_ECN_ECT_0] = 0xff,
 50         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_CE | IPFRAG_ECN_ECT_1] = 0xff,
 51         [IPFRAG_ECN_NOT_ECT | IPFRAG_ECN_CE | IPFRAG_ECN_ECT_0 | IPFRAG_ECN_ECT_1] = 0xff,
 52 };
 53 EXPORT_SYMBOL(ip_frag_ecn_table);
 54 
 55 static unsigned int   //hash函数
 56 inet_frag_hashfn(const struct inet_frags *f, const struct inet_frag_queue *q)
 57 {
 58         return f->hashfn(q) & (INETFRAGS_HASHSZ - 1);
 59 }
 60 
 61 static bool inet_frag_may_rebuild(struct inet_frags *f)
 62 {
 63         return time_after(jiffies,
 64                f->last_rebuild_jiffies + INETFRAGS_MIN_REBUILD_INTERVAL);
 65 }
 66 
 67 static void inet_frag_secret_rebuild(struct inet_frags *f)
 68 {
 69         int i;
 70 
 71         write_seqlock_bh(&f->rnd_seqlock);
 72 
 73         if (!inet_frag_may_rebuild(f))
 74                 goto out;
 75 
 76         get_random_bytes(&f->rnd, sizeof(u32));
 77 
 78         for (i = 0; i < INETFRAGS_HASHSZ; i++) {
 79                 struct inet_frag_bucket *hb;
 80                 struct inet_frag_queue *q;
 81                 struct hlist_node *n;
 82 
 83                 hb = &f->hash[i];
 84                 spin_lock(&hb->chain_lock);
 85 
 86                 hlist_for_each_entry_safe(q, n, &hb->chain, list) {//安全遍历所有hash冲突链表
 87                         unsigned int hval = inet_frag_hashfn(f, q);  //计算hash值
 88 
 89                         if (hval != i) {
 90                                 struct inet_frag_bucket *hb_dest;
 91 
 92                                 hlist_del(&q->list);
 93 
 94                                 /* Relink to new hash chain. */
 95                                 hb_dest = &f->hash[hval];  //hash冲突链表
 96 
 97                                 /* This is the only place where we take
 98                                  * another chain_lock while already holding
 99                                  * one.  As this will not run concurrently,
100                                  * we cannot deadlock on hb_dest lock below, if its
101                                  * already locked it will be released soon since
102                                  * other caller cannot be waiting for hb lock
103                                  * that we‘ve taken above.
104                                  */
105                                 spin_lock_nested(&hb_dest->chain_lock,
106                                                  SINGLE_DEPTH_NESTING);

　　　　　　　　　　　　　　　　　　　　　//把节点list加入到hash冲突链表chain里面
107                                 hlist_add_head(&q->list, &hb_dest->chain);
108                                 spin_unlock(&hb_dest->chain_lock);
109                         }
110                 }
111                 spin_unlock(&hb->chain_lock);
112         }
113 
114         f->rebuild = false;
115         f->last_rebuild_jiffies = jiffies;
116 out:
117         write_sequnlock_bh(&f->rnd_seqlock);
118 }
119 
120 static bool inet_fragq_should_evict(const struct inet_frag_queue *q)
121 {
122         return q->net->low_thresh == 0 ||
123                frag_mem_limit(q->net) >= q->net->low_thresh;
124 }
125 
126 static unsigned int
127 inet_evict_bucket(struct inet_frags *f, struct inet_frag_bucket *hb)
128 {
129         struct inet_frag_queue *fq;
130         struct hlist_node *n;
131         unsigned int evicted = 0;
132         HLIST_HEAD(expired);
133 
134         spin_lock(&hb->chain_lock);
135 
136         hlist_for_each_entry_safe(fq, n, &hb->chain, list) {
137                 if (!inet_fragq_should_evict(fq))
138                         continue;
139 
140                 if (!del_timer(&fq->timer))
141                         continue;
142 
143                 hlist_add_head(&fq->list_evictor, &expired);
144                 ++evicted;
145         }
146 
147         spin_unlock(&hb->chain_lock);
148 
149         hlist_for_each_entry_safe(fq, n, &expired, list_evictor)
150                 f->frag_expire((unsigned long) fq);
151 
152         return evicted;
153 }
154 
155 static void inet_frag_worker(struct work_struct *work)
156 {
157         unsigned int budget = INETFRAGS_EVICT_BUCKETS;
158         unsigned int i, evicted = 0;
159         struct inet_frags *f;
160 
161         f = container_of(work, struct inet_frags, frags_work);
162 
163         BUILD_BUG_ON(INETFRAGS_EVICT_BUCKETS >= INETFRAGS_HASHSZ);
164 
165         local_bh_disable();
166 
167         for (i = ACCESS_ONCE(f->next_bucket); budget; --budget) {
168                 evicted += inet_evict_bucket(f, &f->hash[i]);
169                 i = (i + 1) & (INETFRAGS_HASHSZ - 1);
170                 if (evicted > INETFRAGS_EVICT_MAX)
171                         break;
172         }
173 
174         f->next_bucket = i;
175 
176         local_bh_enable();
177 
178         if (f->rebuild && inet_frag_may_rebuild(f))
179                 inet_frag_secret_rebuild(f);
180 }
181 
182 static void inet_frag_schedule_worker(struct inet_frags *f)
183 {
184         if (unlikely(!work_pending(&f->frags_work)))
185                 schedule_work(&f->frags_work);
186 }
187 
188 int inet_frags_init(struct inet_frags *f)
189 {
190         int i;
191 
192         INIT_WORK(&f->frags_work, inet_frag_worker);
193 
194         for (i = 0; i < INETFRAGS_HASHSZ; i++) { //初始化hash数组中所有的元素
195                 struct inet_frag_bucket *hb = &f->hash[i];
196 
197                 spin_lock_init(&hb->chain_lock);  //初始化一个hash冲突链表
198                 INIT_HLIST_HEAD(&hb->chain);
199         }
200 
201         seqlock_init(&f->rnd_seqlock);
202         f->last_rebuild_jiffies = 0;
203         f->frags_cachep = kmem_cache_create(f->frags_cache_name, f->qsize, 0, 0,
204                                             NULL);
205         if (!f->frags_cachep)
206                 return -ENOMEM;
207 
208         return 0;
209 }
210 EXPORT_SYMBOL(inet_frags_init);
211 
212 void inet_frags_init_net(struct netns_frags *nf)
213 {
214         init_frag_mem_limit(nf);
215 }
216 EXPORT_SYMBOL(inet_frags_init_net);
217 
218 void inet_frags_fini(struct inet_frags *f)
219 {
220         cancel_work_sync(&f->frags_work);
221         kmem_cache_destroy(f->frags_cachep);
222 }
223 EXPORT_SYMBOL(inet_frags_fini);
224 
225 void inet_frags_exit_net(struct netns_frags *nf, struct inet_frags *f)
226 {
227         unsigned int seq;
228         int i;
229 
230         nf->low_thresh = 0;
231 
232 evict_again:
233         local_bh_disable();
234         seq = read_seqbegin(&f->rnd_seqlock);
235 
236         for (i = 0; i < INETFRAGS_HASHSZ ; i++)
237                 inet_evict_bucket(f, &f->hash[i]);
238 
239         local_bh_enable();
240         cond_resched();
241 
242         if (read_seqretry(&f->rnd_seqlock, seq) ||
243             percpu_counter_sum(&nf->mem))
244                 goto evict_again;
245 
246         percpu_counter_destroy(&nf->mem);
247 }
248 EXPORT_SYMBOL(inet_frags_exit_net);
249 
250 static struct inet_frag_bucket *
251 get_frag_bucket_locked(struct inet_frag_queue *fq, struct inet_frags *f)
252 __acquires(hb->chain_lock)
253 {
254         struct inet_frag_bucket *hb;
255         unsigned int seq, hash;
256 
257  restart:
258         seq = read_seqbegin(&f->rnd_seqlock);
259 
260         hash = inet_frag_hashfn(f, fq);
261         hb = &f->hash[hash];
262 
263         spin_lock(&hb->chain_lock);
264         if (read_seqretry(&f->rnd_seqlock, seq)) {
265                 spin_unlock(&hb->chain_lock);
266                 goto restart;
267         }
268 
269         return hb;
270 }
271 
272 static inline void fq_unlink(struct inet_frag_queue *fq, struct inet_frags *f)
273 {
274         struct inet_frag_bucket *hb;
275 
276         hb = get_frag_bucket_locked(fq, f);
277         hlist_del(&fq->list);
278         fq->flags |= INET_FRAG_COMPLETE;
279         spin_unlock(&hb->chain_lock);
280 }
281 
282 void inet_frag_kill(struct inet_frag_queue *fq, struct inet_frags *f)
283 {
284         if (del_timer(&fq->timer))
285                 atomic_dec(&fq->refcnt);
286 
287         if (!(fq->flags & INET_FRAG_COMPLETE)) {
288                 fq_unlink(fq, f);
289                 atomic_dec(&fq->refcnt);
290         }
291 }
292 EXPORT_SYMBOL(inet_frag_kill);
293 
294 static inline void frag_kfree_skb(struct netns_frags *nf, struct inet_frags *f,
295                                   struct sk_buff *skb)
296 {
297         if (f->skb_free)
298                 f->skb_free(skb);
299         kfree_skb(skb);
300 }
301 
302 void inet_frag_destroy(struct inet_frag_queue *q, struct inet_frags *f)
303 {
304         struct sk_buff *fp;
305         struct netns_frags *nf;
306         unsigned int sum, sum_truesize = 0;
307 
308         WARN_ON(!(q->flags & INET_FRAG_COMPLETE));
309         WARN_ON(del_timer(&q->timer) != 0);
310 
311         /* Release all fragment data. */
312         fp = q->fragments;
313         nf = q->net;
314         while (fp) {
315                 struct sk_buff *xp = fp->next;
316 
317                 sum_truesize += fp->truesize;
318                 frag_kfree_skb(nf, f, fp);
319                 fp = xp;
320         }
321         sum = sum_truesize + f->qsize;
322 
323         if (f->destructor)
324                 f->destructor(q);
325         kmem_cache_free(f->frags_cachep, q);
326 
327         sub_frag_mem_limit(nf, sum);
328 }
329 EXPORT_SYMBOL(inet_frag_destroy);
330 
331 static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
332                                                 struct inet_frag_queue *qp_in,
333                                                 struct inet_frags *f,
334                                                 void *arg)
335 {
336         struct inet_frag_bucket *hb = get_frag_bucket_locked(qp_in, f);
337         struct inet_frag_queue *qp;
338 
339 #ifdef CONFIG_SMP
340         /* With SMP race we have to recheck hash table, because
341          * such entry could have been created on other cpu before
342          * we acquired hash bucket lock.
343          */
344         hlist_for_each_entry(qp, &hb->chain, list) {
345                 if (qp->net == nf && f->match(qp, arg)) {
346                         atomic_inc(&qp->refcnt);
347                         spin_unlock(&hb->chain_lock);
348                         qp_in->flags |= INET_FRAG_COMPLETE;
349                         inet_frag_put(qp_in, f);
350                         return qp;
351                 }
352         }
353 #endif
354         qp = qp_in;
355         if (!mod_timer(&qp->timer, jiffies + nf->timeout))
356                 atomic_inc(&qp->refcnt);
357 
358         atomic_inc(&qp->refcnt);
359         hlist_add_head(&qp->list, &hb->chain);
360 
361         spin_unlock(&hb->chain_lock);
362 
363         return qp;
364 }
365 
366 static struct inet_frag_queue *inet_frag_alloc(struct netns_frags *nf,
367                                                struct inet_frags *f,
368                                                void *arg)
369 {
370         struct inet_frag_queue *q;
371 
372         if (frag_mem_limit(nf) > nf->high_thresh) {
373                 inet_frag_schedule_worker(f);
374                 return NULL;
375         }
376 
377         q = kmem_cache_zalloc(f->frags_cachep, GFP_ATOMIC);
378         if (!q)
379                 return NULL;
380 
381         q->net = nf;
382         f->constructor(q, arg);
383         add_frag_mem_limit(nf, f->qsize);
384 
385         setup_timer(&q->timer, f->frag_expire, (unsigned long)q);
386         spin_lock_init(&q->lock);
387         atomic_set(&q->refcnt, 1);
388 
389         return q;
390 }
391 
392 static struct inet_frag_queue *inet_frag_create(struct netns_frags *nf,
393                                                 struct inet_frags *f,
394                                                 void *arg)
395 {
396         struct inet_frag_queue *q;
397 
398         q = inet_frag_alloc(nf, f, arg);
399         if (!q)
400                 return NULL;
401 
402         return inet_frag_intern(nf, q, f, arg);
403 }
404 
405 struct inet_frag_queue *inet_frag_find(struct netns_frags *nf,
406                                        struct inet_frags *f, void *key,
407                                        unsigned int hash)
408 {
409         struct inet_frag_bucket *hb;
410         struct inet_frag_queue *q;
411         int depth = 0;
412 
413         if (frag_mem_limit(nf) > nf->low_thresh)
414                 inet_frag_schedule_worker(f);
415 
416         hash &= (INETFRAGS_HASHSZ - 1);
417         hb = &f->hash[hash];
418 
419         spin_lock(&hb->chain_lock);
420         hlist_for_each_entry(q, &hb->chain, list) {
421                 if (q->net == nf && f->match(q, key)) {
422                         atomic_inc(&q->refcnt);
423                         spin_unlock(&hb->chain_lock);
424                         return q;
425                 }
426                 depth++;
427         }
428         spin_unlock(&hb->chain_lock);
429 
430         if (depth <= INETFRAGS_MAXDEPTH)
431                 return inet_frag_create(nf, f, key);
432 
433         if (inet_frag_may_rebuild(f)) {
434                 if (!f->rebuild)
435                         f->rebuild = true;
436                 inet_frag_schedule_worker(f);
437         }
438 
439         return ERR_PTR(-ENOBUFS);
440 }
441 EXPORT_SYMBOL(inet_frag_find);
442 
443 void inet_frag_maybe_warn_overflow(struct inet_frag_queue *q,
444                                    const char *prefix)
445 {
446         static const char msg[] = "inet_frag_find: Fragment hash bucket"
447                 " list length grew over limit " __stringify(INETFRAGS_MAXDEPTH)
448                 ". Dropping fragment.\n";
449 
450         if (PTR_ERR(q) == -ENOBUFS)
451                 net_dbg_ratelimited("%s%s", prefix, msg);
452 }
453 EXPORT_SYMBOL(inet_frag_maybe_warn_overflow);
454