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Plan 9 has a very interesting synchronization mechanism, the rendezvous() call. A good property of this is that you don't need to explicitly initialize and destroy a barrier object, unlike as with e.g. POSIX barriers (which are mandatory to begin with). Upon "meeting", they can exchange a value. This mechanism will be nice to synchronize certain stages of initialization between threads in the following commit. Unlike Plan 9 rendezvous(), this is not implemented with a hashtable, because that would require additional effort (especially if you want to make it actually scele). Unlike the Plan 9 variant, we use intptr_t instead of void* as type for the value, because I expect that we will be mostly passing a status code as value and not a pointer. Converting an integer to void* requires two cast (because the integer needs to be intptr_t), the other way around it's only one cast. We don't particularly care about performance in this case either. It's simply not important for our use-case. So a simple linked list is used for waiters, and on wakeup, all waiters are temporarily woken up.
55 lines
1.8 KiB
C
55 lines
1.8 KiB
C
#include <pthread.h>
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#include "rendezvous.h"
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static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
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static pthread_cond_t wakeup = PTHREAD_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 return 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: http://9atom.org/magic/man2html/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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pthread_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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pthread_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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pthread_cond_wait(&wakeup, &lock);
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done:
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pthread_mutex_unlock(&lock);
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return value;
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}
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