.\" .\" 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. .\" .Dd March 25, 2024 .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_enter_preempt , .Nm epoch_exit_preempt , .Nm epoch_wait_preempt , .Nm epoch_call , .Nm epoch_drain_callbacks , .Nm in_epoch , .Nm in_epoch_verbose .Nd kernel epoch based reclamation .Sh SYNOPSIS .In sys/param.h .In sys/proc.h .In sys/epoch.h .\" Types .Bd -literal struct epoch; /* Opaque */ .Ed .Vt typedef "struct epoch *epoch_t" ; .Bd -literal struct epoch_context { void *data[2]; }; .Ed .Vt typedef "struct epoch_context *epoch_context_t" ; .Vt typedef "void epoch_callback_t(epoch_context_t)" ; .Bd -literal struct epoch_tracker; /* Opaque */ .Ed .Vt typedef "struct epoch_tracker *epoch_tracker_t" ; .\" Declarations .Ft epoch_t .Fn epoch_alloc "const char *name" "int flags" .Ft void .Fn epoch_free "epoch_t epoch" .Ft void .Fn epoch_enter "epoch_t epoch" .Ft void .Fn epoch_exit "epoch_t epoch" .Ft void .Fn epoch_wait "epoch_t epoch" .Ft void .Fn epoch_enter_preempt "epoch_t epoch" "epoch_tracker_t et" .Ft void .Fn epoch_exit_preempt "epoch_t epoch" "epoch_tracker_t et" .Ft void .Fn epoch_wait_preempt "epoch_t epoch" .Ft void .Fn epoch_call "epoch_t epoch" "epoch_callback_t callback" "epoch_context_t ctx" .Ft void .Fn epoch_drain_callbacks "epoch_t epoch" .Ft int .Fn in_epoch "epoch_t epoch" .Ft int .Fn in_epoch_verbose "epoch_t epoch" "int dump_onfail" .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 . The .Fa name argument is used for debugging convenience when the .Cd EPOCH_TRACE kernel option is configured. By default, epochs do not allow preemption during sections. By default mutexes cannot be held across .Fn epoch_wait_preempt . The .Fa flags specified are formed by .Em OR Ns 'ing the following values: .Bl -tag -offset indent -width Ds .It Dv EPOCH_LOCKED Permit holding mutexes across .Fn epoch_wait_preempt (requires .Dv EPOCH_PREEMPT ) . When doing this one must be cautious of creating a situation where a deadlock is possible. .It Dv EPOCH_PREEMPT The .Vt epoch will allow preemption during sections. Only non-sleepable locks may be acquired during a preemptible epoch. The functions .Fn epoch_enter_preempt , .Fn epoch_exit_preempt , and .Fn epoch_wait_preempt must be used in place of .Fn epoch_enter , .Fn epoch_exit , and .Fn epoch_wait , respectively. .El .Pp .Vt epoch Ns s are freed with .Fn epoch_free . .Pp Threads indicate the start of an epoch critical section by calling .Fn epoch_enter (or .Fn epoch_enter_preempt for preemptible epochs). Threads call .Fn epoch_exit (or .Fn epoch_exit_preempt for preemptible epochs) to indicate the end of a critical section. .Vt struct epoch_tracker Ns s are stack objects whose pointers are passed to .Fn epoch_enter_preempt and .Fn epoch_exit_preempt (much like .Vt struct rm_priotracker ) . .Pp Threads can defer work until a grace period has expired since any thread has entered the epoch either synchronously or asynchronously. .Fn epoch_call defers work asynchronously by invoking the provided .Fa callback at a later time. .Fn epoch_wait (or .Fn epoch_wait_preempt ) blocks the current thread until the grace period has expired and the work can be done safely. .Pp Default, non-preemptible epoch wait .Fn ( epoch_wait ) is guaranteed to have much shorter completion times relative to preemptible epoch wait .Fn ( epoch_wait_preempt ) . (In the default type, none of the threads in an epoch section will be preempted before completing its section.) .Pp INVARIANTS can assert that a thread is in an epoch by using .Fn in_epoch . .Fn in_epoch "epoch" is equivalent to invoking .Fn in_epoch_verbose "epoch" "0" . If .Cd EPOCH_TRACE is enabled, .Fn in_epoch_verbose "epoch" "1" provides additional verbose debugging information. .Pp The epoch API currently does not support sleeping in epoch_preempt sections. A caller should never call .Fn epoch_wait in the middle of an epoch section for the same epoch as this will lead to a deadlock. .Pp The .Fn epoch_drain_callbacks function is used to drain all pending callbacks which have been invoked by prior .Fn epoch_call function calls on the same epoch. This function is useful when there are shared memory structure(s) referred to by the epoch callback(s) which are not refcounted and are rarely freed. The typical place for calling this function is right before freeing or invalidating the shared resource(s) used by the epoch callback(s). This function can sleep and is not optimized for performance. .Sh RETURN VALUES .Fn in_epoch curepoch will return 1 if curthread is in curepoch, 0 otherwise. .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_destroy, &ifa->ifa_epoch_ctx); } 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 SEE ALSO .Xr callout 9 , .Xr locking 9 , .Xr mtx_pool 9 , .Xr mutex 9 , .Xr rwlock 9 , .Xr sema 9 , .Xr sleep 9 , .Xr sx 9 .Sh HISTORY The .Nm framework first appeared in .Fx 11.0 . .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 indefinitely. .Pp 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.