/*-
* Copyright (c) 2016 The NetBSD Foundation, Inc.
* All rights reserved.
*
* This code is derived from software contributed to The NetBSD Foundation
* by Taylor R. Campbell.
*
* 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, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, 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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
* ``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 FOUNDATION OR CONTRIBUTORS
* 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.
*/
/*
* Passive references
*
* Passive references are references to objects that guarantee the
* object will not be destroyed until the reference is released.
*
* Passive references require no interprocessor synchronization to
* acquire or release. However, destroying the target of passive
* references requires expensive interprocessor synchronization --
* xcalls to determine on which CPUs the object is still in use.
*
* Passive references may be held only on a single CPU and by a
* single LWP. They require the caller to allocate a little stack
* space, a struct psref object. Sleeping while a passive
* reference is held is allowed, provided that the owner's LWP is
* bound to a CPU -- e.g., the owner is a softint or a bound
* kthread. However, sleeping should be kept to a short duration,
* e.g. sleeping on an adaptive lock.
*
* Passive references serve as an intermediate stage between
* reference counting and passive serialization (pserialize(9)):
*
* - If you need references to transfer from CPU to CPU or LWP to
* LWP, or if you need long-term references, you must use
* reference counting, e.g. with atomic operations or locks,
* which incurs interprocessor synchronization for every use --
* cheaper than an xcall, but not scalable.
*
* - If all users *guarantee* that they will not sleep, then it is
* not necessary to use passive references: you may as well just
* use the even cheaper pserialize(9), because you have
* satisfied the requirements of a pserialize read section.
*/
/*
* struct psref_class
*
* Private global state for a class of passive reference targets.
* Opaque to callers.
*/
struct psref_class {
kmutex_t prc_lock;
kcondvar_t prc_cv;
struct percpu *prc_percpu; /* struct psref_cpu */
ipl_cookie_t prc_iplcookie;
unsigned int prc_xc_flags;
};
/*
* struct psref_cpu
*
* Private per-CPU state for a class of passive reference targets.
* Not exposed by the API.
*/
struct psref_cpu {
struct psref_head pcpu_head;
};
/*
* Data structures and functions for debugging.
*/
#ifndef PSREF_DEBUG_NITEMS
#define PSREF_DEBUG_NITEMS 16
#endif
/*
* psref_class_create(name, ipl)
*
* Create a new passive reference class, with the given wchan name
* and ipl.
*/
struct psref_class *
psref_class_create(const char *name, int ipl)
{
struct psref_class *class;
/*
* psref_class_destroy(class)
*
* Destroy a passive reference class and free memory associated
* with it. All targets in this class must have been drained and
* destroyed already.
*/
void
psref_class_destroy(struct psref_class *class)
{
/*
* psref_target_init(target, class)
*
* Initialize a passive reference target in the specified class.
* The caller is responsible for issuing a membar_producer after
* psref_target_init and before exposing a pointer to the target
* to other CPUs.
*/
void
psref_target_init(struct psref_target *target,
struct psref_class *class)
{
found = psref_exist(pcpu, psref);
if (!found) {
panic("The psref isn't in the list (releasing unused psref?): "
"psref=%p target=%p", psref, target);
}
}
#endif /* DEBUG */
/*
* psref_acquire(psref, target, class)
*
* Acquire a passive reference to the specified target, which must
* be in the specified class.
*
* The caller must guarantee that the target will not be destroyed
* before psref_acquire returns.
*
* The caller must additionally guarantee that it will not switch
* CPUs before releasing the passive reference, either by
* disabling kpreemption and avoiding sleeps, or by being in a
* softint or in an LWP bound to a CPU.
*/
void
psref_acquire(struct psref *psref, const struct psref_target *target,
struct psref_class *class)
{
struct psref_cpu *pcpu;
int s;
KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() ||
ISSET(curlwp->l_pflag, LP_BOUND)),
"passive references are CPU-local,"
" but preemption is enabled and the caller is not"
" in a softint or CPU-bound LWP");
KASSERTMSG(!target->prt_draining, "psref target already destroyed: %p",
target);
KASSERTMSG((target->prt_class == class),
"mismatched psref target class: %p (ref) != %p (expected)",
target->prt_class, class);
/* Block interrupts and acquire the current CPU's reference list. */
s = splraiseipl(class->prc_iplcookie);
pcpu = percpu_getref(class->prc_percpu);
#ifdef DEBUG
/* Sanity-check if the target is already acquired with the same psref. */
psref_check_duplication(pcpu, psref, target);
#endif
/* Record our reference. */
SLIST_INSERT_HEAD(&pcpu->pcpu_head, psref, psref_entry);
psref->psref_target = target;
psref->psref_lwp = curlwp;
psref->psref_cpu = curcpu();
/* Release the CPU list and restore interrupts. */
percpu_putref(class->prc_percpu);
splx(s);
/*
* psref_release(psref, target, class)
*
* Release a passive reference to the specified target, which must
* be in the specified class.
*
* The caller must not have switched CPUs or LWPs since acquiring
* the passive reference.
*/
void
psref_release(struct psref *psref, const struct psref_target *target,
struct psref_class *class)
{
struct psref_cpu *pcpu;
int s;
KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() ||
ISSET(curlwp->l_pflag, LP_BOUND)),
"passive references are CPU-local,"
" but preemption is enabled and the caller is not"
" in a softint or CPU-bound LWP");
KASSERTMSG((target->prt_class == class),
"mismatched psref target class: %p (ref) != %p (expected)",
target->prt_class, class);
/* Make sure the psref looks sensible. */
KASSERTMSG((psref->psref_target == target),
"passive reference target mismatch: %p (ref) != %p (expected)",
psref->psref_target, target);
KASSERTMSG((psref->psref_lwp == curlwp),
"passive reference transferred from lwp %p to lwp %p",
psref->psref_lwp, curlwp);
KASSERTMSG((psref->psref_cpu == curcpu()),
"passive reference transferred from CPU %u to CPU %u",
cpu_index(psref->psref_cpu), cpu_index(curcpu()));
/*
* Block interrupts and remove the psref from the current CPU's
* list. No need to percpu_getref or get the head of the list,
* and the caller guarantees that we are bound to a CPU anyway
* (as does blocking interrupts).
*/
s = splraiseipl(class->prc_iplcookie);
pcpu = percpu_getref(class->prc_percpu);
#ifdef DEBUG
/* Sanity-check if the target is surely acquired before. */
psref_check_existence(pcpu, psref, target);
#endif
SLIST_REMOVE(&pcpu->pcpu_head, psref, psref, psref_entry);
percpu_putref(class->prc_percpu);
splx(s);
/* If someone is waiting for users to drain, notify 'em. */
if (__predict_false(target->prt_draining))
cv_broadcast(&class->prc_cv);
}
/*
* psref_copy(pto, pfrom, class)
*
* Copy a passive reference from pfrom, which must be in the
* specified class, to pto. Both pfrom and pto must later be
* released with psref_release.
*
* The caller must not have switched CPUs or LWPs since acquiring
* pfrom, and must not switch CPUs or LWPs before releasing both
* pfrom and pto.
*/
void
psref_copy(struct psref *pto, const struct psref *pfrom,
struct psref_class *class)
{
struct psref_cpu *pcpu;
int s;
KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() ||
ISSET(curlwp->l_pflag, LP_BOUND)),
"passive references are CPU-local,"
" but preemption is enabled and the caller is not"
" in a softint or CPU-bound LWP");
KASSERTMSG((pto != pfrom),
"can't copy passive reference to itself: %p",
pto);
/* Make sure the pfrom reference looks sensible. */
KASSERTMSG((pfrom->psref_lwp == curlwp),
"passive reference transferred from lwp %p to lwp %p",
pfrom->psref_lwp, curlwp);
KASSERTMSG((pfrom->psref_cpu == curcpu()),
"passive reference transferred from CPU %u to CPU %u",
cpu_index(pfrom->psref_cpu), cpu_index(curcpu()));
KASSERTMSG((pfrom->psref_target->prt_class == class),
"mismatched psref target class: %p (ref) != %p (expected)",
pfrom->psref_target->prt_class, class);
/* Block interrupts and acquire the current CPU's reference list. */
s = splraiseipl(class->prc_iplcookie);
pcpu = percpu_getref(class->prc_percpu);
/* Record the new reference. */
SLIST_INSERT_HEAD(&pcpu->pcpu_head, pto, psref_entry);
pto->psref_target = pfrom->psref_target;
pto->psref_lwp = curlwp;
pto->psref_cpu = curcpu();
/* Release the CPU list and restore interrupts. */
percpu_putref(class->prc_percpu);
splx(s);
/*
* If we hold a psref to the target, then answer true.
*
* This is the only dynamic decision that may be made with
* psref_held.
*
* No need to lock anything here: every write transitions from
* false to true, so there can be no conflicting writes. No
* need for a memory barrier here because P->ret is read only
* after xc_wait, which has already issued any necessary memory
* barriers.
*/
if (_psref_held(P->target, P->class, true))
P->ret = true;
}
if (__predict_true(mp_online)) {
/*
* Ask all CPUs to say whether they hold a psref to the
* target.
*/
xc_wait(xc_broadcast(class->prc_xc_flags, &psreffed_p_xc, &P,
NULL));
} else
psreffed_p_xc(&P, NULL);
return P.ret;
}
/*
* psref_target_destroy(target, class)
*
* Destroy a passive reference target. Waits for all existing
* references to drain. Caller must guarantee no new references
* will be acquired once it calls psref_target_destroy, e.g. by
* removing the target from a global list first. May sleep.
*/
void
psref_target_destroy(struct psref_target *target, struct psref_class *class)
{
/* Request psref_release to notify us when done. */
target->prt_draining = true;
/* Wait until there are no more references on any CPU. */
while (psreffed_p(target, class)) {
/*
* This enter/wait/exit business looks wrong, but it is
* both necessary, because psreffed_p performs a
* low-priority xcall and hence cannot run while a
* mutex is locked, and OK, because the wait is timed
* -- explicit wakeups are only an optimization.
*/
mutex_enter(&class->prc_lock);
(void)cv_timedwait(&class->prc_cv, &class->prc_lock, 1);
mutex_exit(&class->prc_lock);
}
/* No more references. Cause subsequent psref_acquire to kassert. */
target->prt_class = NULL;
}
KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() ||
ISSET(curlwp->l_pflag, LP_BOUND)),
"passive references are CPU-local,"
" but preemption is enabled and the caller is not"
" in a softint or CPU-bound LWP");
KASSERTMSG((target->prt_class == class),
"mismatched psref target class: %p (ref) != %p (expected)",
target->prt_class, class);
/* Block interrupts and acquire the current CPU's reference list. */
s = splraiseipl(class->prc_iplcookie);
pcpu = percpu_getref(class->prc_percpu);
/* Search through all the references on this CPU. */
SLIST_FOREACH(psref, &pcpu->pcpu_head, psref_entry) {
/* Sanity-check the reference's CPU. */
KASSERTMSG((psref->psref_cpu == curcpu()),
"passive reference transferred from CPU %u to CPU %u",
cpu_index(psref->psref_cpu), cpu_index(curcpu()));
/* If it doesn't match, skip it and move on. */
if (psref->psref_target != target)
continue;
/*
* Sanity-check the reference's LWP if we are asserting
* via psref_held that this LWP holds it, but not if we
* are testing in psref_target_destroy whether any LWP
* still holds it.
*/
KASSERTMSG((lwp_mismatch_ok || psref->psref_lwp == curlwp),
"passive reference transferred from lwp %p to lwp %p",
psref->psref_lwp, curlwp);
/* Stop here and report that we found it. */
held = true;
break;
}
/* Release the CPU list and restore interrupts. */
percpu_putref(class->prc_percpu);
splx(s);
return held;
}
/*
* psref_held(target, class)
*
* True if the current CPU holds a passive reference to target,
* false otherwise. May be used only inside assertions.
*/
bool
psref_held(const struct psref_target *target, struct psref_class *class)
{
