.\" .\" Copyright (C) 2018 Matthew Macy . .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice(s), this list of conditions and the following disclaimer as .\" the first lines of this file unmodified other than the possible .\" addition of one or more copyright notices. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice(s), this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY .\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE .\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY .\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES .\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR .\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER .\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH .\" DAMAGE. .\" .\" $FreeBSD$ .\" .Dd June 25, 2018 .Dt EPOCH 9 .Os .Sh NAME .Nm epoch , .Nm epoch_context , .Nm epoch_alloc , .Nm epoch_free , .Nm epoch_enter , .Nm epoch_exit , .Nm epoch_wait , .Nm epoch_call , .Nm in_epoch , .Nd kernel epoch based reclamation .Sh SYNOPSIS .In sys/param.h .In sys/proc.h .In sys/epoch.h .Ft epoch_t .Fn epoch_alloc "int flags" .Ft void .Fn epoch_enter "epoch_t epoch" .Ft void .Fn epoch_enter_preempt "epoch_t epoch" .Ft void .Fn epoch_exit "epoch_t epoch" .Ft void .Fn epoch_exit_preempt "epoch_t epoch" .Ft void .Fn epoch_wait "epoch_t epoch" .Ft void .Fn epoch_wait_preempt "epoch_t epoch" .Ft void .Fn epoch_call "epoch_t epoch" "epoch_context_t ctx" "void (*callback) (epoch_context_t)" .Ft int .Fn in_epoch "void" .Sh DESCRIPTION Epochs are used to guarantee liveness and immutability of data by deferring reclamation and mutation until a grace period has elapsed. Epochs do not have any lock ordering issues. Entering and leaving an epoch section will never block. .Pp Epochs are allocated with .Fn epoch_alloc and freed with .Fn epoch_free . The flags passed to epoch_alloc determine whether preemption is allowed during a section or not (the default), as specified by EPOCH_PREEMPT. Threads indicate the start of an epoch critical section by calling .Fn epoch_enter . The end of a critical section is indicated by calling .Fn epoch_exit . The _preempt variants can be used around code which requires preemption. A thread can wait until a grace period has elapsed since any threads have entered the epoch by calling .Fn epoch_wait or .Fn epoch_wait_preempt , depending on the epoch_type. The use of a default epoch type allows one to use .Fn epoch_wait which is guaranteed to have much shorter completion times since we know that none of the threads in an epoch section will be preempted before completing its section. If the thread can't sleep or is otherwise in a performance sensitive path it can ensure that a grace period has elapsed by calling .Fn epoch_call with a callback with any work that needs to wait for an epoch to elapse. Only non-sleepable locks can be acquired during a section protected by .Fn epoch_enter_preempt and .Fn epoch_exit_preempt . INVARIANTS can assert that a thread is in an epoch by using .Fn in_epoch . .Pp The epoch API currently does not support sleeping in epoch_preempt sections. A caller cannot do epoch_enter recursively on different preemptible epochs. A caller should never call .Fn epoch_wait in the middle of an epoch section as this will lead to a deadlock. .Pp Note that epochs are not a straight replacement for read locks. Callers must use safe list and tailq traversal routines in an epoch (see ck_queue). When modifying a list referenced from an epoch section safe removal routines must be used and the caller can no longer modify a list entry in place. An item to be modified must be handled with copy on write and frees must be deferred until after a grace period has elapsed. .Sh RETURN VALUES .Fn in_epoch will return 1 if curthread is in an epoch, 0 otherwise. .Sh CAVEATS One must be cautious when using .Fn epoch_wait_preempt threads are pinned during epoch sections so if a thread in a section is then preempted by a higher priority compute bound thread on that CPU it can be prevented from leaving the section. Thus the wait time for the waiter is potentially unbounded. .Sh EXAMPLES Async free example: Thread 1: .Bd -literal int in_pcbladdr(struct inpcb *inp, struct in_addr *faddr, struct in_laddr *laddr, struct ucred *cred) { /* ... */ epoch_enter(net_epoch); CK_STAILQ_FOREACH(ifa, &ifp->if_addrhead, ifa_link) { sa = ifa->ifa_addr; if (sa->sa_family != AF_INET) continue; sin = (struct sockaddr_in *)sa; if (prison_check_ip4(cred, &sin->sin_addr) == 0) { ia = (struct in_ifaddr *)ifa; break; } } epoch_exit(net_epoch); /* ... */ } .Ed Thread 2: .Bd -literal void ifa_free(struct ifaddr *ifa) { if (refcount_release(&ifa->ifa_refcnt)) epoch_call(net_epoch, &ifa->ifa_epoch_ctx, ifa_destroy); } void if_purgeaddrs(struct ifnet *ifp) { /* .... * IF_ADDR_WLOCK(ifp); CK_STAILQ_REMOVE(&ifp->if_addrhead, ifa, ifaddr, ifa_link); IF_ADDR_WUNLOCK(ifp); ifa_free(ifa); } .Ed .Pp Thread 1 traverses the ifaddr list in an epoch. Thread 2 unlinks with the corresponding epoch safe macro, marks as logically free, and then defers deletion. More general mutation or a synchronous free would have to follow a call to .Fn epoch_wait . .Sh ERRORS None. .El .Sh SEE ALSO .Xr locking 9 , .Xr mtx_pool 9 , .Xr mutex 9 , .Xr rwlock 9 , .Xr sema 9 , .Xr sleep 9 , .Xr sx 9 , .Xr timeout 9