mpv/misc/dispatch.c

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/*
* This file is part of mpv.
*
* mpv is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* mpv is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with mpv. If not, see <http://www.gnu.org/licenses/>.
*/
#include <stdbool.h>
#include <assert.h>
#include "common/common.h"
#include "osdep/threads.h"
#include "osdep/timer.h"
#include "dispatch.h"
struct mp_dispatch_queue {
struct mp_dispatch_item *head, *tail;
pthread_mutex_t lock;
pthread_cond_t cond;
int suspend_requested;
bool suspended;
void (*wakeup_fn)(void *wakeup_ctx);
void *wakeup_ctx;
// This lock grant access to the target thread's state during suspend mode.
// During suspend mode, the target thread is blocked in the function
// mp_dispatch_queue_process(), however this function may be processing
// dispatch queue items. This lock serializes the dispatch queue processing
// and external mp_dispatch_lock() calls.
// Invariant: can be held only while suspended==true, and suspend_requested
// must be >0 (unless mp_dispatch_queue_process() locks it). In particular,
// suspend mode must not be left while the lock is held.
pthread_mutex_t exclusive_lock;
};
struct mp_dispatch_item {
mp_dispatch_fn fn;
void *fn_data;
bool asynchronous;
bool completed;
struct mp_dispatch_item *next;
};
static void queue_dtor(void *p)
{
struct mp_dispatch_queue *queue = p;
assert(!queue->head);
assert(!queue->suspend_requested);
assert(!queue->suspended);
pthread_cond_destroy(&queue->cond);
pthread_mutex_destroy(&queue->lock);
pthread_mutex_destroy(&queue->exclusive_lock);
}
// A dispatch queue lets other threads runs callbacks in a target thread.
// The target thread is the thread which created the queue and which calls
// mp_dispatch_queue_process().
// Free the dispatch queue with talloc_free(). (It must be empty.)
struct mp_dispatch_queue *mp_dispatch_create(void *ta_parent)
{
struct mp_dispatch_queue *queue = talloc_ptrtype(ta_parent, queue);
*queue = (struct mp_dispatch_queue){0};
talloc_set_destructor(queue, queue_dtor);
pthread_mutex_init(&queue->exclusive_lock, NULL);
pthread_mutex_init(&queue->lock, NULL);
pthread_cond_init(&queue->cond, NULL);
return queue;
}
// Set a custom function that should be called to guarantee that the target
// thread wakes up. This is intended for use with code that needs to block
// on non-pthread primitives, such as e.g. select(). In the case of select(),
// the wakeup_fn could for example write a byte into a "wakeup" pipe in order
// to unblock the select(). The wakeup_fn is called from the dispatch queue
// when there are new dispatch items, and the target thread should then enter
// mp_dispatch_queue_process() as soon as possible. Note that wakeup_fn is
// called under no lock, so you might have to do synchronization yourself.
void mp_dispatch_set_wakeup_fn(struct mp_dispatch_queue *queue,
void (*wakeup_fn)(void *wakeup_ctx),
void *wakeup_ctx)
{
queue->wakeup_fn = wakeup_fn;
queue->wakeup_ctx = wakeup_ctx;
}
static void mp_dispatch_append(struct mp_dispatch_queue *queue,
struct mp_dispatch_item *item)
{
pthread_mutex_lock(&queue->lock);
if (queue->tail) {
queue->tail->next = item;
} else {
queue->head = item;
}
queue->tail = item;
// Wake up the main thread; note that other threads might wait on this
// condition for reasons, so broadcast the condition.
pthread_cond_broadcast(&queue->cond);
pthread_mutex_unlock(&queue->lock);
if (queue->wakeup_fn)
queue->wakeup_fn(queue->wakeup_ctx);
}
// Enqueue a callback to run it on the target thread asynchronously. The target
// thread will run fn(fn_data) as soon as it enter mp_dispatch_queue_process.
// Note that mp_dispatch_enqueue() will usually return long before that happens.
// It's up to the user to signal completion of the callback. It's also up to
// the user to guarantee that the context fn_data has correct lifetime, i.e.
// lives until the callback is run, and is freed after that.
void mp_dispatch_enqueue(struct mp_dispatch_queue *queue,
mp_dispatch_fn fn, void *fn_data)
{
struct mp_dispatch_item *item = talloc_ptrtype(NULL, item);
*item = (struct mp_dispatch_item){
.fn = fn,
.fn_data = fn_data,
.asynchronous = true,
};
mp_dispatch_append(queue, item);
}
// Like mp_dispatch_enqueue(), but the queue code will call talloc_free(fn_data)
// after the fn callback has been run. (The callback could trivially do that
// itself, but it makes it easier to implement synchronous and asynchronous
// requests with the same callback implementation.)
void mp_dispatch_enqueue_autofree(struct mp_dispatch_queue *queue,
mp_dispatch_fn fn, void *fn_data)
{
struct mp_dispatch_item *item = talloc_ptrtype(NULL, item);
*item = (struct mp_dispatch_item){
.fn = fn,
.fn_data = talloc_steal(item, fn_data),
.asynchronous = true,
};
mp_dispatch_append(queue, item);
}
// Run fn(fn_data) on the target thread synchronously. This function enqueues
// the callback and waits until the target thread is done doing this.
// This is redundant to calling the function inside mp_dispatch_[un]lock(),
// but can be helpful with code that relies on TLS (such as OpenGL).
void mp_dispatch_run(struct mp_dispatch_queue *queue,
mp_dispatch_fn fn, void *fn_data)
{
struct mp_dispatch_item item = {
.fn = fn,
.fn_data = fn_data,
};
mp_dispatch_append(queue, &item);
pthread_mutex_lock(&queue->lock);
while (!item.completed)
pthread_cond_wait(&queue->cond, &queue->lock);
pthread_mutex_unlock(&queue->lock);
}
// Process any outstanding dispatch items in the queue. This also handles
// suspending or locking the target thread.
// The timeout specifies the minimum wait time. The actual time spent in this
// function can be much higher if the suspending/locking functions are used, or
// if executing the dispatch items takes time. On the other hand, this function
// can return much earlier than the timeout due to sporadic wakeups.
// It is also guaranteed that if at least one queue item was processed, the
// function will return as soon as possible, ignoring the timeout. This
// simplifies users, such as re-checking conditions before waiting. (It will
// still process the remaining queue items, and wait for unsuspend.)
void mp_dispatch_queue_process(struct mp_dispatch_queue *queue, double timeout)
{
int64_t wait = timeout > 0 ? mp_add_timeout(mp_time_us(), timeout) : 0;
pthread_mutex_lock(&queue->lock);
queue->suspended = true;
// Wake up thread which called mp_dispatch_suspend().
pthread_cond_broadcast(&queue->cond);
while (queue->head || queue->suspend_requested || wait > 0) {
if (queue->head) {
struct mp_dispatch_item *item = queue->head;
queue->head = item->next;
if (!queue->head)
queue->tail = NULL;
item->next = NULL;
// Unlock, because we want to allow other threads to queue items
// while the dispatch item is processed.
// At the same time, exclusive_lock must be held to protect the
// thread's user state.
pthread_mutex_unlock(&queue->lock);
pthread_mutex_lock(&queue->exclusive_lock);
item->fn(item->fn_data);
pthread_mutex_unlock(&queue->exclusive_lock);
pthread_mutex_lock(&queue->lock);
if (item->asynchronous) {
talloc_free(item);
} else {
item->completed = true;
// Wakeup mp_dispatch_run()
pthread_cond_broadcast(&queue->cond);
}
} else {
if (wait > 0) {
struct timespec ts = mp_time_us_to_timespec(wait);
pthread_cond_timedwait(&queue->cond, &queue->lock, &ts);
} else {
pthread_cond_wait(&queue->cond, &queue->lock);
}
}
wait = 0;
}
queue->suspended = false;
pthread_mutex_unlock(&queue->lock);
}
// Set the target thread into suspend mode: in this mode, the thread will enter
// mp_dispatch_queue_process(), process any outstanding dispatch items, and
// wait for new items when done (instead of exiting the process function).
// Multiple threads can enter suspend mode at the same time. Suspend mode is
// not a synchronization mechanism; it merely makes sure the target thread does
// not leave mp_dispatch_queue_process(), even if it's done. mp_dispatch_lock()
// can be used for exclusive access.
void mp_dispatch_suspend(struct mp_dispatch_queue *queue)
{
pthread_mutex_lock(&queue->lock);
queue->suspend_requested++;
while (!queue->suspended) {
pthread_mutex_unlock(&queue->lock);
if (queue->wakeup_fn)
queue->wakeup_fn(queue->wakeup_ctx);
pthread_mutex_lock(&queue->lock);
if (queue->suspended)
break;
pthread_cond_wait(&queue->cond, &queue->lock);
}
pthread_mutex_unlock(&queue->lock);
}
// Undo mp_dispatch_suspend().
void mp_dispatch_resume(struct mp_dispatch_queue *queue)
{
pthread_mutex_lock(&queue->lock);
assert(queue->suspended);
assert(queue->suspend_requested > 0);
queue->suspend_requested--;
if (queue->suspend_requested == 0)
pthread_cond_broadcast(&queue->cond);
pthread_mutex_unlock(&queue->lock);
}
// Grant exclusive access to the target thread's state. While this is active,
// no other thread can return from mp_dispatch_lock() (i.e. it behaves like
// a pthread mutex), and no other thread can get dispatch items completed.
// Other threads can still queue asynchronous dispatch items without waiting,
// and the mutex behavior applies to this function only.
void mp_dispatch_lock(struct mp_dispatch_queue *queue)
{
mp_dispatch_suspend(queue);
pthread_mutex_lock(&queue->exclusive_lock);
}
// Undo mp_dispatch_lock().
void mp_dispatch_unlock(struct mp_dispatch_queue *queue)
{
pthread_mutex_unlock(&queue->exclusive_lock);
mp_dispatch_resume(queue);
}