From 6c1dac87ff835a48a067fe75bd0a6965921dac78 Mon Sep 17 00:00:00 2001 From: Elizabeth Figura Date: Fri, 13 Dec 2024 13:34:45 -0600 Subject: ntsync: Introduce NTSYNC_IOC_WAIT_ALL. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- drivers/misc/ntsync.c | 336 ++++++++++++++++++++++++++++++++++-- include/uapi/linux/ntsync.h | 1 + 2 files changed, 323 insertions(+), 14 deletions(-) --- a/drivers/misc/ntsync.c +++ b/drivers/misc/ntsync.c @@ -13,6 +13,7 @@ #include #include #include +#include #include #include #include @@ -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 ntsy { __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 ntsy 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 nts 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_o 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 * 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_devi 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_devi 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 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 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 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 */ @@ -406,13 +629,94 @@ static int ntsync_wait_any(struct ntsync 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); + } + + 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_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; @@ -438,6 +742,8 @@ static int ntsync_char_open(struct inode 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 fil 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: --- 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)