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42 .Nd kernel epoch based reclamation
48 .Fn epoch_alloc "int flags"
50 .Fn epoch_enter "epoch_t epoch"
52 .Fn epoch_enter_preempt "epoch_t epoch" "epoch_tracker_t et"
54 .Fn epoch_exit "epoch_t epoch"
56 .Fn epoch_exit_preempt "epoch_t epoch" "epoch_tracker_t et"
58 .Fn epoch_wait "epoch_t epoch"
60 .Fn epoch_wait_preempt "epoch_t epoch"
62 .Fn epoch_call "epoch_t epoch" "epoch_context_t ctx" "void (*callback) (epoch_context_t)"
64 .Fn in_epoch "epoch_t epoch"
66 Epochs are used to guarantee liveness and immutability of data by
67 deferring reclamation and mutation until a grace period has elapsed.
68 Epochs do not have any lock ordering issues.
69 Entering and leaving an epoch section will never block.
71 Epochs are allocated with
75 The flags passed to epoch_alloc determine whether preemption is
76 allowed during a section or not (the default), as specified by
78 Threads indicate the start of an epoch critical section by calling
80 The end of a critical section is indicated by calling
82 The _preempt variants can be used around code which requires preemption.
83 A thread can wait until a grace period has elapsed
84 since any threads have entered
88 .Fn epoch_wait_preempt ,
89 depending on the epoch_type.
90 The use of a default epoch type allows one to use
92 which is guaranteed to have much shorter completion times since
93 we know that none of the threads in an epoch section will be preempted
94 before completing its section.
95 If the thread can't sleep or is otherwise in a performance sensitive
96 path it can ensure that a grace period has elapsed by calling
98 with a callback with any work that needs to wait for an epoch to elapse.
99 Only non-sleepable locks can be acquired during a section protected by
100 .Fn epoch_enter_preempt
102 .Fn epoch_exit_preempt .
103 INVARIANTS can assert that a thread is in an epoch by using
106 The epoch API currently does not support sleeping in epoch_preempt sections.
107 A caller should never call
109 in the middle of an epoch section for the same epoch as this will lead to a deadlock.
111 By default mutexes cannot be held across
112 .Fn epoch_wait_preempt .
113 To permit this the epoch must be allocated with
115 When doing this one must be cautious of creating a situation where a deadlock is
116 possible. Note that epochs are not a straight replacement for read locks.
117 Callers must use safe list and tailq traversal routines in an epoch (see ck_queue).
118 When modifying a list referenced from an epoch section safe removal
119 routines must be used and the caller can no longer modify a list entry
121 An item to be modified must be handled with copy on write
122 and frees must be deferred until after a grace period has elapsed.
124 .Fn in_epoch curepoch
125 will return 1 if curthread is in curepoch, 0 otherwise.
127 One must be cautious when using
128 .Fn epoch_wait_preempt
129 threads are pinned during epoch sections so if a thread in a section is then
130 preempted by a higher priority compute bound thread on that CPU it can be
131 prevented from leaving the section.
132 Thus the wait time for the waiter is
133 potentially unbounded.
139 in_pcbladdr(struct inpcb *inp, struct in_addr *faddr, struct in_laddr *laddr,
143 epoch_enter(net_epoch);
144 CK_STAILQ_FOREACH(ifa, &ifp->if_addrhead, ifa_link) {
146 if (sa->sa_family != AF_INET)
148 sin = (struct sockaddr_in *)sa;
149 if (prison_check_ip4(cred, &sin->sin_addr) == 0) {
150 ia = (struct in_ifaddr *)ifa;
154 epoch_exit(net_epoch);
161 ifa_free(struct ifaddr *ifa)
164 if (refcount_release(&ifa->ifa_refcnt))
165 epoch_call(net_epoch, &ifa->ifa_epoch_ctx, ifa_destroy);
169 if_purgeaddrs(struct ifnet *ifp)
174 CK_STAILQ_REMOVE(&ifp->if_addrhead, ifa, ifaddr, ifa_link);
175 IF_ADDR_WUNLOCK(ifp);
180 Thread 1 traverses the ifaddr list in an epoch.
181 Thread 2 unlinks with the corresponding epoch safe macro, marks as logically free,
182 and then defers deletion.
183 More general mutation or a synchronous
184 free would have to follow a call to