man/man9/epoch.9

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.Dd June 28, 2019
.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 epoch_drain_callbacks ,
.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" "epoch_tracker_t et"
.Ft void
.Fn epoch_exit "epoch_t epoch"
.Ft void
.Fn epoch_exit_preempt "epoch_t epoch" "epoch_tracker_t et"
.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 void
.Fn epoch_drain_callbacks "epoch_t epoch"
.Ft int
.Fn in_epoch "epoch_t epoch"
.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 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
By default mutexes cannot be held across
.Fn epoch_wait_preempt .
To permit this the epoch must be allocated with
EPOCH_LOCKED.
When doing this one must be cautious of creating a situation where a deadlock is
possible. 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.
.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 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.
.Sh NOTES
The
.Nm
kernel programming interface is under development and is subject to change.
.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