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！
以IP分片时hlist使用为例，说明一下hlist的使用。
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 <[email protected]>
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
时间： 2024-10-19 01:16:04