mirror of https://github.com/mpv-player/mpv
56 lines
1.7 KiB
C
56 lines
1.7 KiB
C
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#include "rendezvous.h"
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#include "osdep/threads.h"
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static mp_static_mutex lock = MP_STATIC_MUTEX_INITIALIZER;
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static mp_cond wakeup = MP_STATIC_COND_INITIALIZER;
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static struct waiter *waiters;
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struct waiter {
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void *tag;
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struct waiter *next;
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intptr_t *value;
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};
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/* A barrier for 2 threads, which can exchange a value when they meet.
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* The first thread to call this function will block. As soon as two threads
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* are calling this function with the same tag value, they will unblock, and
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* on each thread the call returns the value parameter of the _other_ thread.
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*
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* tag is an arbitrary value, but it must be an unique pointer. If there are
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* more than 2 threads using the same tag, things won't work. Typically, it
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* will have to point to a memory allocation or to the stack, while pointing
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* it to static data is always a bug.
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*
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* This shouldn't be used for performance critical code (uses a linked list
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* of _all_ waiters in the process, and temporarily wakes up _all_ waiters on
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* each second call).
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*
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* This is inspired by: https://man.cat-v.org/plan_9/2/rendezvous */
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intptr_t mp_rendezvous(void *tag, intptr_t value)
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{
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struct waiter wait = { .tag = tag, .value = &value };
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mp_mutex_lock(&lock);
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struct waiter **prev = &waiters;
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while (*prev) {
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if ((*prev)->tag == tag) {
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intptr_t tmp = *(*prev)->value;
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*(*prev)->value = value;
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value = tmp;
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(*prev)->value = NULL; // signals completion
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*prev = (*prev)->next; // unlink
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mp_cond_broadcast(&wakeup);
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goto done;
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}
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prev = &(*prev)->next;
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}
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*prev = &wait;
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while (wait.value)
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mp_cond_wait(&wakeup, &lock);
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done:
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mp_mutex_unlock(&lock);
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return value;
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}
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