Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Using RCU to Protect Read-Mostly Linked Lists 2 3 4 One of the best applications of RCU is to protect read-mostly linked lists 5 ("struct list_head" in list.h). One big advantage of this approach 6 is that all of the required memory barriers are included for you in 7 the list macros. This document describes several applications of RCU, 8 with the best fits first. 9 10 11 Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates 12 13 The best applications are cases where, if reader-writer locking were 14 used, the read-side lock would be dropped before taking any action 15 based on the results of the search. The most celebrated example is 16 the routing table. Because the routing table is tracking the state of 17 equipment outside of the computer, it will at times contain stale data. 18 Therefore, once the route has been computed, there is no need to hold 19 the routing table static during transmission of the packet. After all, 20 you can hold the routing table static all you want, but that won't keep 21 the external Internet from changing, and it is the state of the external 22 Internet that really matters. In addition, routing entries are typically 23 added or deleted, rather than being modified in place. 24 25 A straightforward example of this use of RCU may be found in the 26 system-call auditing support. For example, a reader-writer locked 27 implementation of audit_filter_task() might be as follows: 28 29 static enum audit_state audit_filter_task(struct task_struct *tsk) 30 { 31 struct audit_entry *e; 32 enum audit_state state; 33 34 read_lock(&auditsc_lock); 35 /* Note: audit_netlink_sem held by caller. */ 36 list_for_each_entry(e, &audit_tsklist, list) { 37 if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { 38 read_unlock(&auditsc_lock); 39 return state; 40 } 41 } 42 read_unlock(&auditsc_lock); 43 return AUDIT_BUILD_CONTEXT; 44 } 45 46 Here the list is searched under the lock, but the lock is dropped before 47 the corresponding value is returned. By the time that this value is acted 48 on, the list may well have been modified. This makes sense, since if 49 you are turning auditing off, it is OK to audit a few extra system calls. 50 51 This means that RCU can be easily applied to the read side, as follows: 52 53 static enum audit_state audit_filter_task(struct task_struct *tsk) 54 { 55 struct audit_entry *e; 56 enum audit_state state; 57 58 rcu_read_lock(); 59 /* Note: audit_netlink_sem held by caller. */ 60 list_for_each_entry_rcu(e, &audit_tsklist, list) { 61 if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { 62 rcu_read_unlock(); 63 return state; 64 } 65 } 66 rcu_read_unlock(); 67 return AUDIT_BUILD_CONTEXT; 68 } 69 70 The read_lock() and read_unlock() calls have become rcu_read_lock() 71 and rcu_read_unlock(), respectively, and the list_for_each_entry() has 72 become list_for_each_entry_rcu(). The _rcu() list-traversal primitives 73 insert the read-side memory barriers that are required on DEC Alpha CPUs. 74 75 The changes to the update side are also straightforward. A reader-writer 76 lock might be used as follows for deletion and insertion: 77 78 static inline int audit_del_rule(struct audit_rule *rule, 79 struct list_head *list) 80 { 81 struct audit_entry *e; 82 83 write_lock(&auditsc_lock); 84 list_for_each_entry(e, list, list) { 85 if (!audit_compare_rule(rule, &e->rule)) { 86 list_del(&e->list); 87 write_unlock(&auditsc_lock); 88 return 0; 89 } 90 } 91 write_unlock(&auditsc_lock); 92 return -EFAULT; /* No matching rule */ 93 } 94 95 static inline int audit_add_rule(struct audit_entry *entry, 96 struct list_head *list) 97 { 98 write_lock(&auditsc_lock); 99 if (entry->rule.flags & AUDIT_PREPEND) { 100 entry->rule.flags &= ~AUDIT_PREPEND; 101 list_add(&entry->list, list); 102 } else { 103 list_add_tail(&entry->list, list); 104 } 105 write_unlock(&auditsc_lock); 106 return 0; 107 } 108 109 Following are the RCU equivalents for these two functions: 110 111 static inline int audit_del_rule(struct audit_rule *rule, 112 struct list_head *list) 113 { 114 struct audit_entry *e; 115 116 /* Do not use the _rcu iterator here, since this is the only 117 * deletion routine. */ 118 list_for_each_entry(e, list, list) { 119 if (!audit_compare_rule(rule, &e->rule)) { 120 list_del_rcu(&e->list); 121 call_rcu(&e->rcu, audit_free_rule); 122 return 0; 123 } 124 } 125 return -EFAULT; /* No matching rule */ 126 } 127 128 static inline int audit_add_rule(struct audit_entry *entry, 129 struct list_head *list) 130 { 131 if (entry->rule.flags & AUDIT_PREPEND) { 132 entry->rule.flags &= ~AUDIT_PREPEND; 133 list_add_rcu(&entry->list, list); 134 } else { 135 list_add_tail_rcu(&entry->list, list); 136 } 137 return 0; 138 } 139 140 Normally, the write_lock() and write_unlock() would be replaced by 141 a spin_lock() and a spin_unlock(), but in this case, all callers hold 142 audit_netlink_sem, so no additional locking is required. The auditsc_lock 143 can therefore be eliminated, since use of RCU eliminates the need for 144 writers to exclude readers. Normally, the write_lock() calls would 145 be converted into spin_lock() calls. 146 147 The list_del(), list_add(), and list_add_tail() primitives have been 148 replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu(). 149 The _rcu() list-manipulation primitives add memory barriers that are 150 needed on weakly ordered CPUs (most of them!). The list_del_rcu() 151 primitive omits the pointer poisoning debug-assist code that would 152 otherwise cause concurrent readers to fail spectacularly. 153 154 So, when readers can tolerate stale data and when entries are either added 155 or deleted, without in-place modification, it is very easy to use RCU! 156 157 158 Example 2: Handling In-Place Updates 159 160 The system-call auditing code does not update auditing rules in place. 161 However, if it did, reader-writer-locked code to do so might look as 162 follows (presumably, the field_count is only permitted to decrease, 163 otherwise, the added fields would need to be filled in): 164 165 static inline int audit_upd_rule(struct audit_rule *rule, 166 struct list_head *list, 167 __u32 newaction, 168 __u32 newfield_count) 169 { 170 struct audit_entry *e; 171 struct audit_newentry *ne; 172 173 write_lock(&auditsc_lock); 174 /* Note: audit_netlink_sem held by caller. */ 175 list_for_each_entry(e, list, list) { 176 if (!audit_compare_rule(rule, &e->rule)) { 177 e->rule.action = newaction; 178 e->rule.file_count = newfield_count; 179 write_unlock(&auditsc_lock); 180 return 0; 181 } 182 } 183 write_unlock(&auditsc_lock); 184 return -EFAULT; /* No matching rule */ 185 } 186 187 The RCU version creates a copy, updates the copy, then replaces the old 188 entry with the newly updated entry. This sequence of actions, allowing 189 concurrent reads while doing a copy to perform an update, is what gives 190 RCU ("read-copy update") its name. The RCU code is as follows: 191 192 static inline int audit_upd_rule(struct audit_rule *rule, 193 struct list_head *list, 194 __u32 newaction, 195 __u32 newfield_count) 196 { 197 struct audit_entry *e; 198 struct audit_newentry *ne; 199 200 list_for_each_entry(e, list, list) { 201 if (!audit_compare_rule(rule, &e->rule)) { 202 ne = kmalloc(sizeof(*entry), GFP_ATOMIC); 203 if (ne == NULL) 204 return -ENOMEM; 205 audit_copy_rule(&ne->rule, &e->rule); 206 ne->rule.action = newaction; 207 ne->rule.file_count = newfield_count; 208 list_replace_rcu(&e->list, &ne->list); 209 call_rcu(&e->rcu, audit_free_rule); 210 return 0; 211 } 212 } 213 return -EFAULT; /* No matching rule */ 214 } 215 216 Again, this assumes that the caller holds audit_netlink_sem. Normally, 217 the reader-writer lock would become a spinlock in this sort of code. 218 219 220 Example 3: Eliminating Stale Data 221 222 The auditing examples above tolerate stale data, as do most algorithms 223 that are tracking external state. Because there is a delay from the 224 time the external state changes before Linux becomes aware of the change, 225 additional RCU-induced staleness is normally not a problem. 226 227 However, there are many examples where stale data cannot be tolerated. 228 One example in the Linux kernel is the System V IPC (see the ipc_lock() 229 function in ipc/util.c). This code checks a "deleted" flag under a 230 per-entry spinlock, and, if the "deleted" flag is set, pretends that the 231 entry does not exist. For this to be helpful, the search function must 232 return holding the per-entry spinlock, as ipc_lock() does in fact do. 233 234 Quick Quiz: Why does the search function need to return holding the 235 per-entry lock for this deleted-flag technique to be helpful? 236 237 If the system-call audit module were to ever need to reject stale data, 238 one way to accomplish this would be to add a "deleted" flag and a "lock" 239 spinlock to the audit_entry structure, and modify audit_filter_task() 240 as follows: 241 242 static enum audit_state audit_filter_task(struct task_struct *tsk) 243 { 244 struct audit_entry *e; 245 enum audit_state state; 246 247 rcu_read_lock(); 248 list_for_each_entry_rcu(e, &audit_tsklist, list) { 249 if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { 250 spin_lock(&e->lock); 251 if (e->deleted) { 252 spin_unlock(&e->lock); 253 rcu_read_unlock(); 254 return AUDIT_BUILD_CONTEXT; 255 } 256 rcu_read_unlock(); 257 return state; 258 } 259 } 260 rcu_read_unlock(); 261 return AUDIT_BUILD_CONTEXT; 262 } 263 264 Note that this example assumes that entries are only added and deleted. 265 Additional mechanism is required to deal correctly with the 266 update-in-place performed by audit_upd_rule(). For one thing, 267 audit_upd_rule() would need additional memory barriers to ensure 268 that the list_add_rcu() was really executed before the list_del_rcu(). 269 270 The audit_del_rule() function would need to set the "deleted" 271 flag under the spinlock as follows: 272 273 static inline int audit_del_rule(struct audit_rule *rule, 274 struct list_head *list) 275 { 276 struct audit_entry *e; 277 278 /* Do not need to use the _rcu iterator here, since this 279 * is the only deletion routine. */ 280 list_for_each_entry(e, list, list) { 281 if (!audit_compare_rule(rule, &e->rule)) { 282 spin_lock(&e->lock); 283 list_del_rcu(&e->list); 284 e->deleted = 1; 285 spin_unlock(&e->lock); 286 call_rcu(&e->rcu, audit_free_rule); 287 return 0; 288 } 289 } 290 return -EFAULT; /* No matching rule */ 291 } 292 293 294 Summary 295 296 Read-mostly list-based data structures that can tolerate stale data are 297 the most amenable to use of RCU. The simplest case is where entries are 298 either added or deleted from the data structure (or atomically modified 299 in place), but non-atomic in-place modifications can be handled by making 300 a copy, updating the copy, then replacing the original with the copy. 301 If stale data cannot be tolerated, then a "deleted" flag may be used 302 in conjunction with a per-entry spinlock in order to allow the search 303 function to reject newly deleted data. 304 305 306 Answer to Quick Quiz 307 Why does the search function need to return holding the per-entry 308 lock for this deleted-flag technique to be helpful? 309 310 If the search function drops the per-entry lock before returning, 311 then the caller will be processing stale data in any case. If it 312 is really OK to be processing stale data, then you don't need a 313 "deleted" flag. If processing stale data really is a problem, 314 then you need to hold the per-entry lock across all of the code 315 that uses the value that was returned.