freebsd-dev/sys/compat/cloudabi/cloudabi_futex.c
Ed Schouten 38526a2cf1 Sync in the latest CloudABI system call definitions.
Some time ago I made a change to merge together the memory scope
definitions used by mmap (MAP_{PRIVATE,SHARED}) and lock objects
(PTHREAD_PROCESS_{PRIVATE,SHARED}). Though that sounded pretty smart
back then, it's backfiring. In the case of mmap it's used with other
flags in a bitmask, but for locking it's an enumeration. As our plan is
to automatically generate bindings for other languages, that looks a bit
sloppy.

Change all of the locking functions to use separate flags instead.

Obtained from:	https://github.com/NuxiNL/cloudabi
2016-03-31 18:50:06 +00:00

1163 lines
32 KiB
C

/*-
* Copyright (c) 2015 Nuxi, https://nuxi.nl/
*
* 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 AUTHOR 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 AUTHOR 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.
*/
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/sx.h>
#include <sys/systm.h>
#include <sys/umtx.h>
#include <contrib/cloudabi/cloudabi_types_common.h>
#include <compat/cloudabi/cloudabi_proto.h>
#include <compat/cloudabi/cloudabi_util.h>
/*
* Futexes for CloudABI.
*
* On most systems, futexes are implemented as objects of a single type
* on which a set of operations can be performed. CloudABI makes a clear
* distinction between locks and condition variables. A lock may have
* zero or more associated condition variables. A condition variable is
* always associated with exactly one lock. There is a strict topology.
* This approach has two advantages:
*
* - This topology is guaranteed to be acyclic. Requeueing of threads
* only happens in one direction (from condition variables to locks).
* This eases locking.
* - It means that a futex object for a lock exists when it is unlocked,
* but has threads waiting on associated condition variables. Threads
* can be requeued to a lock even if the thread performing the wakeup
* does not have the lock mapped in its address space.
*
* This futex implementation only implements a single lock type, namely
* a read-write lock. A regular mutex type would not be necessary, as
* the read-write lock is as efficient as a mutex if used as such.
* Userspace futex locks are 32 bits in size:
*
* - 1 bit: has threads waiting in kernel-space.
* - 1 bit: is write-locked.
* - 30 bits:
* - if write-locked: thread ID of owner.
* - if not write-locked: number of read locks held.
*
* Condition variables are also 32 bits in size. Its value is modified
* by kernel-space exclusively. Zero indicates that it has no waiting
* threads. Non-zero indicates the opposite.
*
* This implementation is optimal, in the sense that it only wakes up
* threads if they can actually continue execution. It does not suffer
* from the thundering herd problem. If multiple threads waiting on a
* condition variable need to be woken up, only a single thread is
* scheduled. All other threads are 'donated' to this thread. After the
* thread manages to reacquire the lock, it requeues its donated threads
* to the lock.
*
* TODO(ed): Integrate this functionality into kern_umtx.c instead.
* TODO(ed): Store futex objects in a hash table.
* TODO(ed): Add actual priority inheritance.
* TODO(ed): Let futex_queue also take priorities into account.
* TODO(ed): Make locking fine-grained.
* TODO(ed): Perform sleeps until an actual absolute point in time,
* instead of converting the timestamp to a relative value.
*/
struct futex_address;
struct futex_condvar;
struct futex_lock;
struct futex_queue;
struct futex_waiter;
/* Identifier of a location in memory. */
struct futex_address {
struct umtx_key fa_key;
};
/* A set of waiting threads. */
struct futex_queue {
STAILQ_HEAD(, futex_waiter) fq_list;
unsigned int fq_count;
};
/* Condition variables. */
struct futex_condvar {
/* Address of the condition variable. */
struct futex_address fc_address;
/* The lock the waiters should be moved to when signalled. */
struct futex_lock * fc_lock;
/* Threads waiting on the condition variable. */
struct futex_queue fc_waiters;
/*
* Number of threads blocked on this condition variable, or
* being blocked on the lock after being requeued.
*/
unsigned int fc_waitcount;
/* Global list pointers. */
LIST_ENTRY(futex_condvar) fc_next;
};
/* Read-write locks. */
struct futex_lock {
/* Address of the lock. */
struct futex_address fl_address;
/*
* Current owner of the lock. LOCK_UNMANAGED if the lock is
* currently not owned by the kernel. LOCK_OWNER_UNKNOWN in case
* the owner is not known (e.g., when the lock is read-locked).
*/
cloudabi_tid_t fl_owner;
#define LOCK_UNMANAGED 0x0
#define LOCK_OWNER_UNKNOWN 0x1
/* Writers blocked on the lock. */
struct futex_queue fl_writers;
/* Readers blocked on the lock. */
struct futex_queue fl_readers;
/* Number of threads blocked on this lock + condition variables. */
unsigned int fl_waitcount;
/* Global list pointers. */
LIST_ENTRY(futex_lock) fl_next;
};
/* Information associated with a thread blocked on an object. */
struct futex_waiter {
/* Thread ID. */
cloudabi_tid_t fw_tid;
/* Condition variable used for waiting. */
struct cv fw_wait;
/* Queue this waiter is currently placed in. */
struct futex_queue * fw_queue;
/* List pointers of fw_queue. */
STAILQ_ENTRY(futex_waiter) fw_next;
/* Lock has been acquired. */
bool fw_locked;
/* If not locked, threads that should block after acquiring. */
struct futex_queue fw_donated;
};
/* Global data structures. */
static MALLOC_DEFINE(M_FUTEX, "futex", "CloudABI futex");
static struct sx futex_global_lock;
SX_SYSINIT(futex_global_lock, &futex_global_lock, "CloudABI futex global lock");
static LIST_HEAD(, futex_lock) futex_lock_list =
LIST_HEAD_INITIALIZER(&futex_lock_list);
static LIST_HEAD(, futex_condvar) futex_condvar_list =
LIST_HEAD_INITIALIZER(&futex_condvar_list);
/* Utility functions. */
static void futex_lock_assert(const struct futex_lock *);
static struct futex_lock *futex_lock_lookup_locked(struct futex_address *);
static void futex_lock_release(struct futex_lock *);
static int futex_lock_tryrdlock(struct futex_lock *, cloudabi_lock_t *);
static int futex_lock_unmanage(struct futex_lock *, cloudabi_lock_t *);
static int futex_lock_update_owner(struct futex_lock *, cloudabi_lock_t *);
static int futex_lock_wake_up_next(struct futex_lock *, cloudabi_lock_t *);
static unsigned int futex_queue_count(const struct futex_queue *);
static void futex_queue_init(struct futex_queue *);
static void futex_queue_requeue(struct futex_queue *, struct futex_queue *,
unsigned int);
static int futex_queue_sleep(struct futex_queue *, struct futex_lock *,
struct futex_waiter *, struct thread *, cloudabi_clockid_t,
cloudabi_timestamp_t, cloudabi_timestamp_t);
static cloudabi_tid_t futex_queue_tid_best(const struct futex_queue *);
static void futex_queue_wake_up_all(struct futex_queue *);
static void futex_queue_wake_up_best(struct futex_queue *);
static void futex_queue_wake_up_donate(struct futex_queue *, unsigned int);
static int futex_user_load(uint32_t *, uint32_t *);
static int futex_user_store(uint32_t *, uint32_t);
static int futex_user_cmpxchg(uint32_t *, uint32_t, uint32_t *, uint32_t);
/*
* futex_address operations.
*/
static int
futex_address_create(struct futex_address *fa, struct thread *td,
const void *object, cloudabi_scope_t scope)
{
KASSERT(td == curthread,
("Can only create umtx keys for the current thread"));
switch (scope) {
case CLOUDABI_SCOPE_PRIVATE:
return (umtx_key_get(object, TYPE_FUTEX, THREAD_SHARE,
&fa->fa_key));
case CLOUDABI_SCOPE_SHARED:
return (umtx_key_get(object, TYPE_FUTEX, AUTO_SHARE,
&fa->fa_key));
default:
return (EINVAL);
}
}
static void
futex_address_free(struct futex_address *fa)
{
umtx_key_release(&fa->fa_key);
}
static bool
futex_address_match(const struct futex_address *fa1,
const struct futex_address *fa2)
{
return (umtx_key_match(&fa1->fa_key, &fa2->fa_key));
}
/*
* futex_condvar operations.
*/
static void
futex_condvar_assert(const struct futex_condvar *fc)
{
KASSERT(fc->fc_waitcount >= futex_queue_count(&fc->fc_waiters),
("Total number of waiters cannot be smaller than the wait queue"));
futex_lock_assert(fc->fc_lock);
}
static int
futex_condvar_lookup(struct thread *td, const cloudabi_condvar_t *address,
cloudabi_scope_t scope, struct futex_condvar **fcret)
{
struct futex_address fa_condvar;
struct futex_condvar *fc;
int error;
error = futex_address_create(&fa_condvar, td, address, scope);
if (error != 0)
return (error);
sx_xlock(&futex_global_lock);
LIST_FOREACH(fc, &futex_condvar_list, fc_next) {
if (futex_address_match(&fc->fc_address, &fa_condvar)) {
/* Found matching lock object. */
futex_address_free(&fa_condvar);
futex_condvar_assert(fc);
*fcret = fc;
return (0);
}
}
sx_xunlock(&futex_global_lock);
futex_address_free(&fa_condvar);
return (ENOENT);
}
static int
futex_condvar_lookup_or_create(struct thread *td,
const cloudabi_condvar_t *condvar, cloudabi_scope_t condvar_scope,
const cloudabi_lock_t *lock, cloudabi_scope_t lock_scope,
struct futex_condvar **fcret)
{
struct futex_address fa_condvar, fa_lock;
struct futex_condvar *fc;
struct futex_lock *fl;
int error;
error = futex_address_create(&fa_condvar, td, condvar, condvar_scope);
if (error != 0)
return (error);
error = futex_address_create(&fa_lock, td, lock, lock_scope);
if (error != 0) {
futex_address_free(&fa_condvar);
return (error);
}
sx_xlock(&futex_global_lock);
LIST_FOREACH(fc, &futex_condvar_list, fc_next) {
if (!futex_address_match(&fc->fc_address, &fa_condvar))
continue;
fl = fc->fc_lock;
if (!futex_address_match(&fl->fl_address, &fa_lock)) {
/* Condition variable is owned by a different lock. */
futex_address_free(&fa_condvar);
futex_address_free(&fa_lock);
sx_xunlock(&futex_global_lock);
return (EINVAL);
}
/* Found fully matching condition variable. */
futex_address_free(&fa_condvar);
futex_address_free(&fa_lock);
futex_condvar_assert(fc);
*fcret = fc;
return (0);
}
/* None found. Create new condition variable object. */
fc = malloc(sizeof(*fc), M_FUTEX, M_WAITOK);
fc->fc_address = fa_condvar;
fc->fc_lock = futex_lock_lookup_locked(&fa_lock);
futex_queue_init(&fc->fc_waiters);
fc->fc_waitcount = 0;
LIST_INSERT_HEAD(&futex_condvar_list, fc, fc_next);
*fcret = fc;
return (0);
}
static void
futex_condvar_release(struct futex_condvar *fc)
{
struct futex_lock *fl;
futex_condvar_assert(fc);
fl = fc->fc_lock;
if (fc->fc_waitcount == 0) {
/* Condition variable has no waiters. Deallocate it. */
futex_address_free(&fc->fc_address);
LIST_REMOVE(fc, fc_next);
free(fc, M_FUTEX);
}
futex_lock_release(fl);
}
static int
futex_condvar_unmanage(struct futex_condvar *fc,
cloudabi_condvar_t *condvar)
{
if (futex_queue_count(&fc->fc_waiters) != 0)
return (0);
return (futex_user_store(condvar, CLOUDABI_CONDVAR_HAS_NO_WAITERS));
}
/*
* futex_lock operations.
*/
static void
futex_lock_assert(const struct futex_lock *fl)
{
/*
* A futex lock can only be kernel-managed if it has waiters.
* Vice versa: if a futex lock has waiters, it must be
* kernel-managed.
*/
KASSERT((fl->fl_owner == LOCK_UNMANAGED) ==
(futex_queue_count(&fl->fl_readers) == 0 &&
futex_queue_count(&fl->fl_writers) == 0),
("Managed locks must have waiting threads"));
KASSERT(fl->fl_waitcount != 0 || fl->fl_owner == LOCK_UNMANAGED,
("Lock with no waiters must be unmanaged"));
}
static int
futex_lock_lookup(struct thread *td, const cloudabi_lock_t *address,
cloudabi_scope_t scope, struct futex_lock **flret)
{
struct futex_address fa;
int error;
error = futex_address_create(&fa, td, address, scope);
if (error != 0)
return (error);
sx_xlock(&futex_global_lock);
*flret = futex_lock_lookup_locked(&fa);
return (0);
}
static struct futex_lock *
futex_lock_lookup_locked(struct futex_address *fa)
{
struct futex_lock *fl;
LIST_FOREACH(fl, &futex_lock_list, fl_next) {
if (futex_address_match(&fl->fl_address, fa)) {
/* Found matching lock object. */
futex_address_free(fa);
futex_lock_assert(fl);
return (fl);
}
}
/* None found. Create new lock object. */
fl = malloc(sizeof(*fl), M_FUTEX, M_WAITOK);
fl->fl_address = *fa;
fl->fl_owner = LOCK_UNMANAGED;
futex_queue_init(&fl->fl_readers);
futex_queue_init(&fl->fl_writers);
fl->fl_waitcount = 0;
LIST_INSERT_HEAD(&futex_lock_list, fl, fl_next);
return (fl);
}
static int
futex_lock_rdlock(struct futex_lock *fl, struct thread *td,
cloudabi_lock_t *lock, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision)
{
struct futex_waiter fw;
int error;
error = futex_lock_tryrdlock(fl, lock);
if (error == EBUSY) {
/* Suspend execution. */
KASSERT(fl->fl_owner != LOCK_UNMANAGED,
("Attempted to sleep on an unmanaged lock"));
error = futex_queue_sleep(&fl->fl_readers, fl, &fw, td,
clock_id, timeout, precision);
KASSERT((error == 0) == fw.fw_locked,
("Should have locked write lock on success"));
KASSERT(futex_queue_count(&fw.fw_donated) == 0,
("Lock functions cannot receive threads"));
}
if (error != 0)
futex_lock_unmanage(fl, lock);
return (error);
}
static void
futex_lock_release(struct futex_lock *fl)
{
futex_lock_assert(fl);
if (fl->fl_waitcount == 0) {
/* Lock object is unreferenced. Deallocate it. */
KASSERT(fl->fl_owner == LOCK_UNMANAGED,
("Attempted to free a managed lock"));
futex_address_free(&fl->fl_address);
LIST_REMOVE(fl, fl_next);
free(fl, M_FUTEX);
}
sx_xunlock(&futex_global_lock);
}
static int
futex_lock_unmanage(struct futex_lock *fl, cloudabi_lock_t *lock)
{
cloudabi_lock_t cmp, old;
int error;
if (futex_queue_count(&fl->fl_readers) == 0 &&
futex_queue_count(&fl->fl_writers) == 0) {
/* Lock should be unmanaged. */
fl->fl_owner = LOCK_UNMANAGED;
/* Clear kernel-managed bit. */
error = futex_user_load(lock, &old);
if (error != 0)
return (error);
for (;;) {
cmp = old;
error = futex_user_cmpxchg(lock, cmp, &old,
cmp & ~CLOUDABI_LOCK_KERNEL_MANAGED);
if (error != 0)
return (error);
if (old == cmp)
break;
}
}
return (0);
}
/* Sets an owner of a lock, based on a userspace lock value. */
static void
futex_lock_set_owner(struct futex_lock *fl, cloudabi_lock_t lock)
{
/* Lock has no explicit owner. */
if ((lock & ~CLOUDABI_LOCK_WRLOCKED) == 0) {
fl->fl_owner = LOCK_OWNER_UNKNOWN;
return;
}
lock &= ~(CLOUDABI_LOCK_WRLOCKED | CLOUDABI_LOCK_KERNEL_MANAGED);
/* Don't allow userspace to silently unlock. */
if (lock == LOCK_UNMANAGED) {
fl->fl_owner = LOCK_OWNER_UNKNOWN;
return;
}
fl->fl_owner = lock;
}
static int
futex_lock_unlock(struct futex_lock *fl, struct thread *td,
cloudabi_lock_t *lock)
{
int error;
/* Validate that this thread is allowed to unlock. */
error = futex_lock_update_owner(fl, lock);
if (error != 0)
return (error);
if (fl->fl_owner != LOCK_UNMANAGED && fl->fl_owner != td->td_tid)
return (EPERM);
return (futex_lock_wake_up_next(fl, lock));
}
/* Syncs in the owner of the lock from userspace if needed. */
static int
futex_lock_update_owner(struct futex_lock *fl, cloudabi_lock_t *address)
{
cloudabi_lock_t lock;
int error;
if (fl->fl_owner == LOCK_OWNER_UNKNOWN) {
error = futex_user_load(address, &lock);
if (error != 0)
return (error);
futex_lock_set_owner(fl, lock);
}
return (0);
}
static int
futex_lock_tryrdlock(struct futex_lock *fl, cloudabi_lock_t *address)
{
cloudabi_lock_t old, cmp;
int error;
if (fl->fl_owner != LOCK_UNMANAGED) {
/* Lock is already acquired. */
return (EBUSY);
}
old = CLOUDABI_LOCK_UNLOCKED;
for (;;) {
if ((old & CLOUDABI_LOCK_KERNEL_MANAGED) != 0) {
/*
* Userspace lock is kernel-managed, even though
* the kernel disagrees.
*/
return (EINVAL);
}
if ((old & CLOUDABI_LOCK_WRLOCKED) == 0) {
/*
* Lock is not write-locked. Attempt to acquire
* it by increasing the read count.
*/
cmp = old;
error = futex_user_cmpxchg(address, cmp, &old, cmp + 1);
if (error != 0)
return (error);
if (old == cmp) {
/* Success. */
return (0);
}
} else {
/* Lock is write-locked. Make it kernel-managed. */
cmp = old;
error = futex_user_cmpxchg(address, cmp, &old,
cmp | CLOUDABI_LOCK_KERNEL_MANAGED);
if (error != 0)
return (error);
if (old == cmp) {
/* Success. */
futex_lock_set_owner(fl, cmp);
return (EBUSY);
}
}
}
}
static int
futex_lock_trywrlock(struct futex_lock *fl, cloudabi_lock_t *address,
cloudabi_tid_t tid, bool force_kernel_managed)
{
cloudabi_lock_t old, new, cmp;
int error;
if (fl->fl_owner == tid) {
/* Attempted to acquire lock recursively. */
return (EDEADLK);
}
if (fl->fl_owner != LOCK_UNMANAGED) {
/* Lock is already acquired. */
return (EBUSY);
}
old = CLOUDABI_LOCK_UNLOCKED;
for (;;) {
if ((old & CLOUDABI_LOCK_KERNEL_MANAGED) != 0) {
/*
* Userspace lock is kernel-managed, even though
* the kernel disagrees.
*/
return (EINVAL);
}
if (old == (tid | CLOUDABI_LOCK_WRLOCKED)) {
/* Attempted to acquire lock recursively. */
return (EDEADLK);
}
if (old == CLOUDABI_LOCK_UNLOCKED) {
/* Lock is unlocked. Attempt to acquire it. */
new = tid | CLOUDABI_LOCK_WRLOCKED;
if (force_kernel_managed)
new |= CLOUDABI_LOCK_KERNEL_MANAGED;
error = futex_user_cmpxchg(address,
CLOUDABI_LOCK_UNLOCKED, &old, new);
if (error != 0)
return (error);
if (old == CLOUDABI_LOCK_UNLOCKED) {
/* Success. */
if (force_kernel_managed)
fl->fl_owner = tid;
return (0);
}
} else {
/* Lock is still locked. Make it kernel-managed. */
cmp = old;
error = futex_user_cmpxchg(address, cmp, &old,
cmp | CLOUDABI_LOCK_KERNEL_MANAGED);
if (error != 0)
return (error);
if (old == cmp) {
/* Success. */
futex_lock_set_owner(fl, cmp);
return (EBUSY);
}
}
}
}
static int
futex_lock_wake_up_next(struct futex_lock *fl, cloudabi_lock_t *lock)
{
cloudabi_tid_t tid;
int error;
/*
* Determine which thread(s) to wake up. Prefer waking up
* writers over readers to prevent write starvation.
*/
if (futex_queue_count(&fl->fl_writers) > 0) {
/* Transfer ownership to a single write-locker. */
if (futex_queue_count(&fl->fl_writers) > 1 ||
futex_queue_count(&fl->fl_readers) > 0) {
/* Lock should remain managed afterwards. */
tid = futex_queue_tid_best(&fl->fl_writers);
error = futex_user_store(lock,
tid | CLOUDABI_LOCK_WRLOCKED |
CLOUDABI_LOCK_KERNEL_MANAGED);
if (error != 0)
return (error);
futex_queue_wake_up_best(&fl->fl_writers);
fl->fl_owner = tid;
} else {
/* Lock can become unmanaged afterwards. */
error = futex_user_store(lock,
futex_queue_tid_best(&fl->fl_writers) |
CLOUDABI_LOCK_WRLOCKED);
if (error != 0)
return (error);
futex_queue_wake_up_best(&fl->fl_writers);
fl->fl_owner = LOCK_UNMANAGED;
}
} else {
/* Transfer ownership to all read-lockers (if any). */
error = futex_user_store(lock,
futex_queue_count(&fl->fl_readers));
if (error != 0)
return (error);
/* Wake up all threads. */
futex_queue_wake_up_all(&fl->fl_readers);
fl->fl_owner = LOCK_UNMANAGED;
}
return (0);
}
static int
futex_lock_wrlock(struct futex_lock *fl, struct thread *td,
cloudabi_lock_t *lock, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision,
struct futex_queue *donated)
{
struct futex_waiter fw;
int error;
error = futex_lock_trywrlock(fl, lock, td->td_tid,
futex_queue_count(donated) > 0);
if (error == 0 || error == EBUSY) {
/* Put donated threads in queue before suspending. */
KASSERT(futex_queue_count(donated) == 0 ||
fl->fl_owner != LOCK_UNMANAGED,
("Lock should be managed if we are going to donate"));
futex_queue_requeue(donated, &fl->fl_writers, UINT_MAX);
} else {
/*
* This thread cannot deal with the donated threads.
* Wake up the next thread and let it try it by itself.
*/
futex_queue_wake_up_donate(donated, UINT_MAX);
}
if (error == EBUSY) {
/* Suspend execution if the lock was busy. */
KASSERT(fl->fl_owner != LOCK_UNMANAGED,
("Attempted to sleep on an unmanaged lock"));
error = futex_queue_sleep(&fl->fl_writers, fl, &fw, td,
clock_id, timeout, precision);
KASSERT((error == 0) == fw.fw_locked,
("Should have locked write lock on success"));
KASSERT(futex_queue_count(&fw.fw_donated) == 0,
("Lock functions cannot receive threads"));
}
if (error != 0)
futex_lock_unmanage(fl, lock);
return (error);
}
/*
* futex_queue operations.
*/
static cloudabi_tid_t
futex_queue_tid_best(const struct futex_queue *fq)
{
return (STAILQ_FIRST(&fq->fq_list)->fw_tid);
}
static unsigned int
futex_queue_count(const struct futex_queue *fq)
{
return (fq->fq_count);
}
static void
futex_queue_init(struct futex_queue *fq)
{
STAILQ_INIT(&fq->fq_list);
fq->fq_count = 0;
}
/* Converts a relative timestamp to an sbintime. */
static sbintime_t
futex_queue_convert_timestamp_relative(cloudabi_timestamp_t ts)
{
cloudabi_timestamp_t s, ns;
s = ts / 1000000000;
ns = ts % 1000000000;
if (s > INT32_MAX)
return (INT64_MAX);
return ((s << 32) + (ns << 32) / 1000000000);
}
/* Converts an absolute timestamp and precision to a pair of sbintime values. */
static int
futex_queue_convert_timestamp(struct thread *td, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision,
sbintime_t *sbttimeout, sbintime_t *sbtprecision)
{
cloudabi_timestamp_t now;
int error;
/* Make the time relative. */
error = cloudabi_clock_time_get(td, clock_id, &now);
if (error != 0)
return (error);
timeout = timeout < now ? 0 : timeout - now;
*sbttimeout = futex_queue_convert_timestamp_relative(timeout);
*sbtprecision = futex_queue_convert_timestamp_relative(precision);
return (0);
}
static int
futex_queue_sleep(struct futex_queue *fq, struct futex_lock *fl,
struct futex_waiter *fw, struct thread *td, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision)
{
sbintime_t sbttimeout, sbtprecision;
int error;
/* Initialize futex_waiter object. */
fw->fw_tid = td->td_tid;
fw->fw_locked = false;
futex_queue_init(&fw->fw_donated);
if (timeout != UINT64_MAX) {
/* Convert timeout duration. */
error = futex_queue_convert_timestamp(td, clock_id, timeout,
precision, &sbttimeout, &sbtprecision);
if (error != 0)
return (error);
}
/* Place object in the queue. */
fw->fw_queue = fq;
STAILQ_INSERT_TAIL(&fq->fq_list, fw, fw_next);
++fq->fq_count;
cv_init(&fw->fw_wait, "futex");
++fl->fl_waitcount;
futex_lock_assert(fl);
if (timeout == UINT64_MAX) {
/* Wait without a timeout. */
error = cv_wait_sig(&fw->fw_wait, &futex_global_lock);
} else {
/* Wait respecting the timeout. */
error = cv_timedwait_sig_sbt(&fw->fw_wait, &futex_global_lock,
sbttimeout, sbtprecision, 0);
futex_lock_assert(fl);
if (error == EWOULDBLOCK &&
fw->fw_queue != NULL && fw->fw_queue != fq) {
/*
* We got signalled on a condition variable, but
* observed a timeout while waiting to reacquire
* the lock. In other words, we didn't actually
* time out. Go back to sleep and wait for the
* lock to be reacquired.
*/
error = cv_wait_sig(&fw->fw_wait, &futex_global_lock);
}
}
futex_lock_assert(fl);
--fl->fl_waitcount;
cv_destroy(&fw->fw_wait);
fq = fw->fw_queue;
if (fq == NULL) {
/* Thread got dequeued, so we've slept successfully. */
return (0);
}
/* Thread is still enqueued. Remove it. */
KASSERT(error != 0, ("Woken up thread is still enqueued"));
STAILQ_REMOVE(&fq->fq_list, fw, futex_waiter, fw_next);
--fq->fq_count;
return (error == EWOULDBLOCK ? ETIMEDOUT : error);
}
/* Moves up to nwaiters waiters from one queue to another. */
static void
futex_queue_requeue(struct futex_queue *fqfrom, struct futex_queue *fqto,
unsigned int nwaiters)
{
struct futex_waiter *fw;
/* Move waiters to the target queue. */
while (nwaiters-- > 0 && !STAILQ_EMPTY(&fqfrom->fq_list)) {
fw = STAILQ_FIRST(&fqfrom->fq_list);
STAILQ_REMOVE_HEAD(&fqfrom->fq_list, fw_next);
--fqfrom->fq_count;
fw->fw_queue = fqto;
STAILQ_INSERT_TAIL(&fqto->fq_list, fw, fw_next);
++fqto->fq_count;
}
}
/* Wakes up all waiters in a queue. */
static void
futex_queue_wake_up_all(struct futex_queue *fq)
{
struct futex_waiter *fw;
STAILQ_FOREACH(fw, &fq->fq_list, fw_next) {
fw->fw_locked = true;
fw->fw_queue = NULL;
cv_signal(&fw->fw_wait);
}
STAILQ_INIT(&fq->fq_list);
fq->fq_count = 0;
}
/*
* Wakes up the best waiter (i.e., the waiter having the highest
* priority) in a queue.
*/
static void
futex_queue_wake_up_best(struct futex_queue *fq)
{
struct futex_waiter *fw;
fw = STAILQ_FIRST(&fq->fq_list);
fw->fw_locked = true;
fw->fw_queue = NULL;
cv_signal(&fw->fw_wait);
STAILQ_REMOVE_HEAD(&fq->fq_list, fw_next);
--fq->fq_count;
}
static void
futex_queue_wake_up_donate(struct futex_queue *fq, unsigned int nwaiters)
{
struct futex_waiter *fw;
fw = STAILQ_FIRST(&fq->fq_list);
if (fw == NULL)
return;
fw->fw_locked = false;
fw->fw_queue = NULL;
cv_signal(&fw->fw_wait);
STAILQ_REMOVE_HEAD(&fq->fq_list, fw_next);
--fq->fq_count;
futex_queue_requeue(fq, &fw->fw_donated, nwaiters);
}
/*
* futex_user operations. Used to adjust values in userspace.
*/
static int
futex_user_load(uint32_t *obj, uint32_t *val)
{
return (fueword32(obj, val) != 0 ? EFAULT : 0);
}
static int
futex_user_store(uint32_t *obj, uint32_t val)
{
return (suword32(obj, val) != 0 ? EFAULT : 0);
}
static int
futex_user_cmpxchg(uint32_t *obj, uint32_t cmp, uint32_t *old, uint32_t new)
{
return (casueword32(obj, cmp, old, new) != 0 ? EFAULT : 0);
}
/*
* Blocking calls: acquiring locks, waiting on condition variables.
*/
int
cloudabi_futex_condvar_wait(struct thread *td, cloudabi_condvar_t *condvar,
cloudabi_scope_t condvar_scope, cloudabi_lock_t *lock,
cloudabi_scope_t lock_scope, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision)
{
struct futex_condvar *fc;
struct futex_lock *fl;
struct futex_waiter fw;
int error, error2;
/* Lookup condition variable object. */
error = futex_condvar_lookup_or_create(td, condvar, condvar_scope, lock,
lock_scope, &fc);
if (error != 0)
return (error);
fl = fc->fc_lock;
/*
* Set the condition variable to something other than
* CLOUDABI_CONDVAR_HAS_NO_WAITERS to make userspace threads
* call into the kernel to perform wakeups.
*/
error = futex_user_store(condvar, ~CLOUDABI_CONDVAR_HAS_NO_WAITERS);
if (error != 0) {
futex_condvar_release(fc);
return (error);
}
/* Drop the lock. */
error = futex_lock_unlock(fl, td, lock);
if (error != 0) {
futex_condvar_unmanage(fc, condvar);
futex_condvar_release(fc);
return (error);
}
/* Go to sleep. */
++fc->fc_waitcount;
error = futex_queue_sleep(&fc->fc_waiters, fc->fc_lock, &fw, td,
clock_id, timeout, precision);
if (fw.fw_locked) {
/* Waited and got the lock assigned to us. */
KASSERT(futex_queue_count(&fw.fw_donated) == 0,
("Received threads while being locked"));
} else if (error == 0 || error == ETIMEDOUT) {
if (error != 0)
futex_condvar_unmanage(fc, condvar);
/*
* Got woken up without having the lock assigned to us.
* This can happen in two cases:
*
* 1. We observed a timeout on a condition variable.
* 2. We got signalled on a condition variable while the
* associated lock is unlocked. We are the first
* thread that gets woken up. This thread is
* responsible for reacquiring the userspace lock.
*/
error2 = futex_lock_wrlock(fl, td, lock,
CLOUDABI_CLOCK_MONOTONIC, UINT64_MAX, 0, &fw.fw_donated);
if (error2 != 0)
error = error2;
} else {
KASSERT(futex_queue_count(&fw.fw_donated) == 0,
("Received threads on error"));
futex_condvar_unmanage(fc, condvar);
futex_lock_unmanage(fl, lock);
}
--fc->fc_waitcount;
futex_condvar_release(fc);
return (error);
}
int
cloudabi_futex_lock_rdlock(struct thread *td, cloudabi_lock_t *lock,
cloudabi_scope_t scope, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision)
{
struct futex_lock *fl;
int error;
/* Look up lock object. */
error = futex_lock_lookup(td, lock, scope, &fl);
if (error != 0)
return (error);
error = futex_lock_rdlock(fl, td, lock, clock_id, timeout,
precision);
futex_lock_release(fl);
return (error);
}
int
cloudabi_futex_lock_wrlock(struct thread *td, cloudabi_lock_t *lock,
cloudabi_scope_t scope, cloudabi_clockid_t clock_id,
cloudabi_timestamp_t timeout, cloudabi_timestamp_t precision)
{
struct futex_lock *fl;
struct futex_queue fq;
int error;
/* Look up lock object. */
error = futex_lock_lookup(td, lock, scope, &fl);
if (error != 0)
return (error);
futex_queue_init(&fq);
error = futex_lock_wrlock(fl, td, lock, clock_id, timeout,
precision, &fq);
futex_lock_release(fl);
return (error);
}
/*
* Non-blocking calls: releasing locks, signalling condition variables.
*/
int
cloudabi_sys_condvar_signal(struct thread *td,
struct cloudabi_sys_condvar_signal_args *uap)
{
struct futex_condvar *fc;
struct futex_lock *fl;
cloudabi_nthreads_t nwaiters;
int error;
nwaiters = uap->nwaiters;
if (nwaiters == 0) {
/* No threads to wake up. */
return (0);
}
/* Look up futex object. */
error = futex_condvar_lookup(td, uap->condvar, uap->scope, &fc);
if (error != 0) {
/* Race condition: condition variable with no waiters. */
return (error == ENOENT ? 0 : error);
}
fl = fc->fc_lock;
if (fl->fl_owner == LOCK_UNMANAGED) {
/*
* The lock is currently not managed by the kernel,
* meaning we must attempt to acquire the userspace lock
* first. We cannot requeue threads to an unmanaged lock,
* as these threads will then never be scheduled.
*
* Unfortunately, the memory address of the lock is
* unknown from this context, meaning that we cannot
* acquire the lock on behalf of the first thread to be
* scheduled. The lock may even not be mapped within the
* address space of the current thread.
*
* To solve this, wake up a single waiter that will
* attempt to acquire the lock. Donate all of the other
* waiters that need to be woken up to this waiter, so
* it can requeue them after acquiring the lock.
*/
futex_queue_wake_up_donate(&fc->fc_waiters, nwaiters - 1);
} else {
/*
* Lock is already managed by the kernel. This makes it
* easy, as we can requeue the threads from the
* condition variable directly to the associated lock.
*/
futex_queue_requeue(&fc->fc_waiters, &fl->fl_writers, nwaiters);
}
/* Clear userspace condition variable if all waiters are gone. */
error = futex_condvar_unmanage(fc, uap->condvar);
futex_condvar_release(fc);
return (error);
}
int
cloudabi_sys_lock_unlock(struct thread *td,
struct cloudabi_sys_lock_unlock_args *uap)
{
struct futex_lock *fl;
int error;
error = futex_lock_lookup(td, uap->lock, uap->scope, &fl);
if (error != 0)
return (error);
error = futex_lock_unlock(fl, td, uap->lock);
futex_lock_release(fl);
return (error);
}