This is similar to NTSYNC_IOC_WAIT_ANY, but waits until all of the objects are
simultaneously signaled, and then acquires all of them as a single atomic
operation.
Because acquisition of multiple objects is atomic, some complex locking is
required. We cannot simply spin-lock multiple objects simultaneously, as that
may disable preëmption for a problematically long time.
Instead, modifying any object which may be involved in a wait-all operation takes
a device-wide sleeping mutex, "wait_all_lock", instead of the normal object
spinlock.
Because wait-for-all is a rare operation, in order to optimize wait-for-any,
this lock is only taken when necessary. "all_hint" is used to mark objects which
are involved in a wait-for-all operation, and if an object is not, only its
spinlock is taken.
The locking scheme used here was written by Peter Zijlstra.
Signed-off-by: Elizabeth Figura <zfigura@xxxxxxxxxxxxxxx>
---
drivers/misc/ntsync.c | 336 ++++++++++++++++++++++++++++++++++--
include/uapi/linux/ntsync.h | 1 +
2 files changed, 323 insertions(+), 14 deletions(-)
diff --git a/drivers/misc/ntsync.c b/drivers/misc/ntsync.c
index 900cc5ce5761..c8beb5504bcc 100644
--- a/drivers/misc/ntsync.c
+++ b/drivers/misc/ntsync.c
@@ -13,6 +13,7 @@
#include <linux/ktime.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
+#include <linux/mutex.h>
#include <linux/overflow.h>
#include <linux/sched.h>
#include <linux/sched/signal.h>
@@ -41,6 +42,7 @@ enum ntsync_type {
struct ntsync_obj {
spinlock_t lock;
+ int dev_locked;
enum ntsync_type type;
@@ -55,7 +57,30 @@ struct ntsync_obj {
} sem;
} u;
+ /*
+ * any_waiters is protected by the object lock, but all_waiters is
+ * protected by the device wait_all_lock.
+ */
struct list_head any_waiters;
+ struct list_head all_waiters;
+
+ /*
+ * Hint describing how many tasks are queued on this object in a
+ * wait-all operation.
+ *
+ * Any time we do a wake, we may need to wake "all" waiters as well as
+ * "any" waiters. In order to atomically wake "all" waiters, we must
+ * lock all of the objects, and that means grabbing the wait_all_lock
+ * below (and, due to lock ordering rules, before locking this object).
+ * However, wait-all is a rare operation, and grabbing the wait-all
+ * lock for every wake would create unnecessary contention.
+ * Therefore we first check whether all_hint is zero, and, if it is,
+ * we skip trying to wake "all" waiters.
+ *
+ * Since wait requests must originate from user-space threads, we're
+ * limited here by PID_MAX_LIMIT, so there's no risk of overflow.
+ */
+ atomic_t all_hint;
};
struct ntsync_q_entry {
@@ -75,19 +100,198 @@ struct ntsync_q {
*/
atomic_t signaled;
+ bool all;
__u32 count;
struct ntsync_q_entry entries[];
};
struct ntsync_device {
+ /*
+ * Wait-all operations must atomically grab all objects, and be totally
+ * ordered with respect to each other and wait-any operations.
+ * If one thread is trying to acquire several objects, another thread
+ * cannot touch the object at the same time.
+ *
+ * This device-wide lock is used to serialize wait-for-all
+ * operations, and operations on an object that is involved in a
+ * wait-for-all.
+ */
+ struct mutex wait_all_lock;
+
struct file *file;
};
+/*
+ * Single objects are locked using obj->lock.
+ *
+ * Multiple objects are 'locked' while holding dev->wait_all_lock.
+ * In this case however, individual objects are not locked by holding
+ * obj->lock, but by setting obj->dev_locked.
+ *
+ * This means that in order to lock a single object, the sequence is slightly
+ * more complicated than usual. Specifically it needs to check obj->dev_locked
+ * after acquiring obj->lock, if set, it needs to drop the lock and acquire
+ * dev->wait_all_lock in order to serialize against the multi-object operation.
+ */
+
+static void dev_lock_obj(struct ntsync_device *dev, struct ntsync_obj *obj)
+{
+ lockdep_assert_held(&dev->wait_all_lock);
+ lockdep_assert(obj->dev == dev);
+ spin_lock(&obj->lock);
+ /*
+ * By setting obj->dev_locked inside obj->lock, it is ensured that
+ * anyone holding obj->lock must see the value.
+ */
+ obj->dev_locked = 1;
+ spin_unlock(&obj->lock);
+}
+
+static void dev_unlock_obj(struct ntsync_device *dev, struct ntsync_obj *obj)
+{
+ lockdep_assert_held(&dev->wait_all_lock);
+ lockdep_assert(obj->dev == dev);
+ spin_lock(&obj->lock);
+ obj->dev_locked = 0;
+ spin_unlock(&obj->lock);
+}
+
+static void obj_lock(struct ntsync_obj *obj)
+{
+ struct ntsync_device *dev = obj->dev;
+
+ for (;;) {
+ spin_lock(&obj->lock);
+ if (likely(!obj->dev_locked))
+ break;
+
+ spin_unlock(&obj->lock);
+ mutex_lock(&dev->wait_all_lock);
+ spin_lock(&obj->lock);
+ /*
+ * obj->dev_locked should be set and released under the same
+ * wait_all_lock section, since we now own this lock, it should
+ * be clear.
+ */
+ lockdep_assert(!obj->dev_locked);
+ spin_unlock(&obj->lock);
+ mutex_unlock(&dev->wait_all_lock);
+ }
+}
+
+static void obj_unlock(struct ntsync_obj *obj)
+{
+ spin_unlock(&obj->lock);
+}
+
+static bool ntsync_lock_obj(struct ntsync_device *dev, struct ntsync_obj *obj)
+{
+ bool all;
+
+ obj_lock(obj);
+ all = atomic_read(&obj->all_hint);
+ if (unlikely(all)) {
+ obj_unlock(obj);
+ mutex_lock(&dev->wait_all_lock);
+ dev_lock_obj(dev, obj);
+ }
+
+ return all;
+}
+
+static void ntsync_unlock_obj(struct ntsync_device *dev, struct ntsync_obj *obj, bool all)
+{
+ if (all) {
+ dev_unlock_obj(dev, obj);
+ mutex_unlock(&dev->wait_all_lock);
+ } else {
+ obj_unlock(obj);
+ }
+}
+
+#define ntsync_assert_held(obj) \
+ lockdep_assert((lockdep_is_held(&(obj)->lock) != LOCK_STATE_NOT_HELD) || \
+ ((lockdep_is_held(&(obj)->dev->wait_all_lock) != LOCK_STATE_NOT_HELD) && \
+ (obj)->dev_locked))
+
+static bool is_signaled(struct ntsync_obj *obj)
+{
+ ntsync_assert_held(obj);
+
+ switch (obj->type) {
+ case NTSYNC_TYPE_SEM:
+ return !!obj->u.sem.count;
+ }
+
+ WARN(1, "bad object type %#x\n", obj->type);
+ return false;
+}
+
+/*
+ * "locked_obj" is an optional pointer to an object which is already locked and
+ * should not be locked again. This is necessary so that changing an object's
+ * state and waking it can be a single atomic operation.
+ */
+static void try_wake_all(struct ntsync_device *dev, struct ntsync_q *q,
+ struct ntsync_obj *locked_obj)
+{
+ __u32 count = q->count;
+ bool can_wake = true;
+ int signaled = -1;
+ __u32 i;
+
+ lockdep_assert_held(&dev->wait_all_lock);
+ if (locked_obj)
+ lockdep_assert(locked_obj->dev_locked);
+
+ for (i = 0; i < count; i++) {
+ if (q->entries[i].obj != locked_obj)
+ dev_lock_obj(dev, q->entries[i].obj);
+ }
+
+ for (i = 0; i < count; i++) {
+ if (!is_signaled(q->entries[i].obj)) {
+ can_wake = false;
+ break;
+ }
+ }
+
+ if (can_wake && atomic_try_cmpxchg(&q->signaled, &signaled, 0)) {
+ for (i = 0; i < count; i++) {
+ struct ntsync_obj *obj = q->entries[i].obj;
+
+ switch (obj->type) {
+ case NTSYNC_TYPE_SEM:
+ obj->u.sem.count--;
+ break;
+ }
+ }
+ wake_up_process(q->task);
+ }
+
+ for (i = 0; i < count; i++) {
+ if (q->entries[i].obj != locked_obj)
+ dev_unlock_obj(dev, q->entries[i].obj);
+ }
+}
+
+static void try_wake_all_obj(struct ntsync_device *dev, struct ntsync_obj *obj)
+{
+ struct ntsync_q_entry *entry;
+
+ lockdep_assert_held(&dev->wait_all_lock);
+ lockdep_assert(obj->dev_locked);
+
+ list_for_each_entry(entry, &obj->all_waiters, node)
+ try_wake_all(dev, entry->q, obj);
+}
+
static void try_wake_any_sem(struct ntsync_obj *sem)
{
struct ntsync_q_entry *entry;
- lockdep_assert_held(&sem->lock);
+ ntsync_assert_held(sem);
+ lockdep_assert(sem->type == NTSYNC_TYPE_SEM);
list_for_each_entry(entry, &sem->any_waiters, node) {
struct ntsync_q *q = entry->q;
@@ -111,7 +315,7 @@ static int release_sem_state(struct ntsync_obj *sem, __u32 count)
{
__u32 sum;
- lockdep_assert_held(&sem->lock);
+ ntsync_assert_held(sem);
if (check_add_overflow(sem->u.sem.count, count, &sum) ||
sum > sem->u.sem.max)
@@ -123,9 +327,11 @@ static int release_sem_state(struct ntsync_obj *sem, __u32 count)
static int ntsync_sem_release(struct ntsync_obj *sem, void __user *argp)
{
+ struct ntsync_device *dev = sem->dev;
__u32 __user *user_args = argp;
__u32 prev_count;
__u32 args;
+ bool all;
int ret;
if (copy_from_user(&args, argp, sizeof(args)))
@@ -134,14 +340,17 @@ static int ntsync_sem_release(struct ntsync_obj *sem, void __user *argp)
if (sem->type != NTSYNC_TYPE_SEM)
return -EINVAL;
- spin_lock(&sem->lock);
+ all = ntsync_lock_obj(dev, sem);
prev_count = sem->u.sem.count;
- ret = post_sem_state(sem, args);
- if (!ret)
+ ret = release_sem_state(sem, args);
+ if (!ret) {
+ if (all)
+ try_wake_all_obj(dev, sem);
try_wake_any_sem(sem);
+ }
- spin_unlock(&sem->lock);
+ ntsync_unlock_obj(dev, sem, all);
if (!ret && put_user(prev_count, user_args))
ret = -EFAULT;
@@ -193,6 +402,8 @@ static struct ntsync_obj *ntsync_alloc_obj(struct ntsync_device *dev,
get_file(dev->file);
spin_lock_init(&obj->lock);
INIT_LIST_HEAD(&obj->any_waiters);
+ INIT_LIST_HEAD(&obj->all_waiters);
+ atomic_set(&obj->all_hint, 0);
return obj;
}
@@ -301,7 +512,7 @@ static int ntsync_schedule(const struct ntsync_q *q, const struct ntsync_wait_ar
* Allocate and initialize the ntsync_q structure, but do not queue us yet.
*/
static int setup_wait(struct ntsync_device *dev,
- const struct ntsync_wait_args *args,
+ const struct ntsync_wait_args *args, bool all,
struct ntsync_q **ret_q)
{
const __u32 count = args->count;
@@ -324,6 +535,7 @@ static int setup_wait(struct ntsync_device *dev,
return -ENOMEM;
q->task = current;
atomic_set(&q->signaled, -1);
+ q->all = all;
q->count = count;
for (i = 0; i < count; i++) {
@@ -333,6 +545,16 @@ static int setup_wait(struct ntsync_device *dev,
if (!obj)
goto err;
+ if (all) {
+ /* Check that the objects are all distinct. */
+ for (j = 0; j < i; j++) {
+ if (obj == q->entries[j].obj) {
+ put_obj(obj);
+ goto err;
+ }
+ }
+ }
+
entry->obj = obj;
entry->q = q;
entry->index = i;
@@ -362,13 +584,14 @@ static int ntsync_wait_any(struct ntsync_device *dev, void __user *argp)
struct ntsync_wait_args args;
struct ntsync_q *q;
int signaled;
+ bool all;
__u32 i;
int ret;
if (copy_from_user(&args, argp, sizeof(args)))
return -EFAULT;
- ret = setup_wait(dev, &args, &q);
+ ret = setup_wait(dev, &args, false, &q);
if (ret < 0)
return ret;
@@ -378,9 +601,9 @@ static int ntsync_wait_any(struct ntsync_device *dev, void __user *argp)
struct ntsync_q_entry *entry = &q->entries[i];
struct ntsync_obj *obj = entry->obj;
- spin_lock(&obj->lock);
+ all = ntsync_lock_obj(dev, obj);
list_add_tail(&entry->node, &obj->any_waiters);
- spin_unlock(&obj->lock);
+ ntsync_unlock_obj(dev, obj, all);
}
/* check if we are already signaled */
@@ -391,9 +614,9 @@ static int ntsync_wait_any(struct ntsync_device *dev, void __user *argp)
if (atomic_read(&q->signaled) != -1)
break;
- spin_lock(&obj->lock);
+ all = ntsync_lock_obj(dev, obj);
try_wake_any_obj(obj);
- spin_unlock(&obj->lock);
+ ntsync_unlock_obj(dev, obj, all);
}
/* sleep */
@@ -406,9 +629,9 @@ static int ntsync_wait_any(struct ntsync_device *dev, void __user *argp)
struct ntsync_q_entry *entry = &q->entries[i];
struct ntsync_obj *obj = entry->obj;
- spin_lock(&obj->lock);
+ all = ntsync_lock_obj(dev, obj);
list_del(&entry->node);
- spin_unlock(&obj->lock);
+ ntsync_unlock_obj(dev, obj, all);
put_obj(obj);
}
@@ -430,6 +653,87 @@ static int ntsync_wait_any(struct ntsync_device *dev, void __user *argp)
return ret;
}
+static int ntsync_wait_all(struct ntsync_device *dev, void __user *argp)
+{
+ struct ntsync_wait_args args;
+ struct ntsync_q *q;
+ int signaled;
+ __u32 i;
+ int ret;
+
+ if (copy_from_user(&args, argp, sizeof(args)))
+ return -EFAULT;
+
+ ret = setup_wait(dev, &args, true, &q);
+ if (ret < 0)
+ return ret;
+
+ /* queue ourselves */
+
+ mutex_lock(&dev->wait_all_lock);
+
+ for (i = 0; i < args.count; i++) {
+ struct ntsync_q_entry *entry = &q->entries[i];
+ struct ntsync_obj *obj = entry->obj;
+
+ atomic_inc(&obj->all_hint);
+
+ /*
+ * obj->all_waiters is protected by dev->wait_all_lock rather
+ * than obj->lock, so there is no need to acquire obj->lock
+ * here.
+ */
+ list_add_tail(&entry->node, &obj->all_waiters);
+ }
+
+ /* check if we are already signaled */
+
+ try_wake_all(dev, q, NULL);
+
+ mutex_unlock(&dev->wait_all_lock);
+
+ /* sleep */
+
+ ret = ntsync_schedule(q, &args);
+
+ /* and finally, unqueue */
+
+ mutex_lock(&dev->wait_all_lock);
+
+ for (i = 0; i < args.count; i++) {
+ struct ntsync_q_entry *entry = &q->entries[i];
+ struct ntsync_obj *obj = entry->obj;
+
+ /*
+ * obj->all_waiters is protected by dev->wait_all_lock rather
+ * than obj->lock, so there is no need to acquire it here.
+ */
+ list_del(&entry->node);
+
+ atomic_dec(&obj->all_hint);
+
+ put_obj(obj);
+ }
+
+ mutex_unlock(&dev->wait_all_lock);
+
+ signaled = atomic_read(&q->signaled);
+ if (signaled != -1) {
+ struct ntsync_wait_args __user *user_args = argp;
+
+ /* even if we caught a signal, we need to communicate success */
+ ret = 0;
+
+ if (put_user(signaled, &user_args->index))
+ ret = -EFAULT;
+ } else if (!ret) {
+ ret = -ETIMEDOUT;
+ }
+
+ kfree(q);
+ return ret;
+}
+
static int ntsync_char_open(struct inode *inode, struct file *file)
{
struct ntsync_device *dev;
@@ -438,6 +742,8 @@ static int ntsync_char_open(struct inode *inode, struct file *file)
if (!dev)
return -ENOMEM;
+ mutex_init(&dev->wait_all_lock);
+
file->private_data = dev;
dev->file = file;
return nonseekable_open(inode, file);
@@ -461,6 +767,8 @@ static long ntsync_char_ioctl(struct file *file, unsigned int cmd,
switch (cmd) {
case NTSYNC_IOC_CREATE_SEM:
return ntsync_create_sem(dev, argp);
+ case NTSYNC_IOC_WAIT_ALL:
+ return ntsync_wait_all(dev, argp);
case NTSYNC_IOC_WAIT_ANY:
return ntsync_wait_any(dev, argp);
default:
diff --git a/include/uapi/linux/ntsync.h b/include/uapi/linux/ntsync.h
index 40ffdc41d5bb..d64420ffbcd7 100644
--- a/include/uapi/linux/ntsync.h
+++ b/include/uapi/linux/ntsync.h
@@ -30,6 +30,7 @@ struct ntsync_wait_args {
#define NTSYNC_IOC_CREATE_SEM _IOW ('N', 0x80, struct ntsync_sem_args)
#define NTSYNC_IOC_WAIT_ANY _IOWR('N', 0x82, struct ntsync_wait_args)
+#define NTSYNC_IOC_WAIT_ALL _IOWR('N', 0x83, struct ntsync_wait_args)
#define NTSYNC_IOC_SEM_RELEASE _IOWR('N', 0x81, __u32)
--
2.45.2
