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apply feedback re: PR #4392

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zeromus 2017-01-21 16:49:10 -06:00
parent f4c187e980
commit 10157c5831
1 changed files with 40 additions and 18 deletions
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58
libretro-common/rthreads/rthreads.c
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@ -92,7 +92,7 @@ struct scond
/* This will be used as a linked list immplementing a queue of waiting threads */ /* This will be used as a linked list immplementing a queue of waiting threads */
struct QueueEntry struct QueueEntry
{ {
struct QueueEntry* next; struct QueueEntry *next;
}; };
/* With this implementation of scond, we don't have any way of waking (or even identifying) specific threads */ /* With this implementation of scond, we don't have any way of waking (or even identifying) specific threads */
@ -102,16 +102,16 @@ struct scond
HANDLE hot_potato; HANDLE hot_potato;
/* The primary signalled event. Hot potatoes are passed until this is set. */ /* The primary signalled event. Hot potatoes are passed until this is set. */
HANDLE event; HANDLE event;
/* the head of the queue; NULL if queue is empty */ /* the head of the queue; NULL if queue is empty */
struct QueueEntry* head; struct QueueEntry *head;
/* equivalent to the queue length */ /* equivalent to the queue length */
int waiters; int waiters;
/* how many waiters in the queue have been conceptually wakened by signals (even if we haven't managed to actually wake them yet */ /* how many waiters in the queue have been conceptually wakened by signals (even if we haven't managed to actually wake them yet */
int wakens; int wakens;
#else #else
pthread_cond_t cond; pthread_cond_t cond;
@ -189,7 +189,7 @@ error:
* @thread : pointer to thread object * @thread : pointer to thread object
* *
* Detach a thread. When a detached thread terminates, its * Detach a thread. When a detached thread terminates, its
* resource sare automatically released back to the system * resources are automatically released back to the system
* without the need for another thread to join with the * without the need for another thread to join with the
* terminated thread. * terminated thread.
* *
@ -351,15 +351,17 @@ scond_t *scond_new(void)
#ifdef USE_WIN32_THREADS #ifdef USE_WIN32_THREADS
/* This is very complex because recreating condition variable semantics with win32 parts is not easy */ /* This is very complex because recreating condition variable semantics with win32 parts is not easy */
/* The main problem is that a condition variable can be used to wake up a thread, but only if the thread is already waiting; */ /* The main problem is that a condition variable can't be used to "pre-wake" a thread (it will get wakened only after it's waited) */
/* whereas a win32 event will 'wake up' a thread in advance (the event will be set in advance, so a 'waiter' wont even have to wait on it) */ /* whereas a win32 event can pre-wake a thread (the event will be set in advance, so a 'waiter' won't even have to wait on it) */
/* Keep in mind a condition variable can apparently pre-wake a thread, insofar as spurious wakeups are always possible, */
/* but nobody will be expecting this and it does not need to be simulated. */
/* So at the very least, we need to do something clever. But there's bigger problems. */ /* So at the very least, we need to do something clever. But there's bigger problems. */
/* We don't even have a straightforward way in win32 to satisfy pthread_cond_wait's atomicity requirement. The bulk of this algorithm is solving that. */ /* We don't even have a straightforward way in win32 to satisfy pthread_cond_wait's atomicity requirement. The bulk of this algorithm is solving that. */
/* Note: We might could simplify this using vista+ condition variables, but we wanted an XP compatible solution. */ /* Note: We might could simplify this using vista+ condition variables, but we wanted an XP compatible solution. */
cond->event = CreateEvent(NULL, FALSE, FALSE, NULL); cond->event = CreateEvent(NULL, FALSE, FALSE, NULL);
if(!cond->event) goto error; if (!cond->event) goto error;
cond->hot_potato = CreateEvent(NULL, FALSE, FALSE, NULL); cond->hot_potato = CreateEvent(NULL, FALSE, FALSE, NULL);
if(!cond->hot_potato) if (!cond->hot_potato)
{ {
CloseHandle(cond->event); CloseHandle(cond->event);
goto error; goto error;
@ -410,9 +412,11 @@ void scond_wait(scond_t *cond, slock_t *lock)
{ {
#ifdef USE_WIN32_THREADS #ifdef USE_WIN32_THREADS
/* Reminder: `lock` is held before this is called */
/* add ourselves to a queue of waiting threads */ /* add ourselves to a queue of waiting threads */
struct QueueEntry myentry; struct QueueEntry myentry;
struct QueueEntry** ptr = &cond->head; struct QueueEntry **ptr = &cond->head;
while(*ptr) /* walk to the end of the linked list */ while(*ptr) /* walk to the end of the linked list */
ptr = &((*ptr)->next); ptr = &((*ptr)->next);
*ptr = &myentry; *ptr = &myentry;
@ -431,18 +435,36 @@ void scond_wait(scond_t *cond, slock_t *lock)
/* It's my turn if I'm the head of the queue. Check to see if it's my turn. */ /* It's my turn if I'm the head of the queue. Check to see if it's my turn. */
while (cond->head != &myentry) while (cond->head != &myentry)
{ {
/* It isn't my turn: */
/* As long as someone is even going to be able to wake up when they receive the potato, keep it going round */ /* As long as someone is even going to be able to wake up when they receive the potato, keep it going round */
if (cond->wakens > 0) if (cond->wakens > 0)
SetEvent(cond->hot_potato); SetEvent(cond->hot_potato);
/* Wait to catch the hot potato before checking for my turn again */ /* Let someone else go */
SignalObjectAndWait(lock->lock, cond->hot_potato, INFINITE, FALSE); ReleaseMutex(lock->lock);
slock_lock(lock);
/* Wait a while to catch the hot potato.. someone else should get a chance to go */
/* After all, it isn't my turn (and it must be someone else's */
Sleep(0);
WaitForSingleObject(cond->hot_potato, INFINITE);
/* I should come out of here with the main lock taken */
WaitForSingleObject(lock->lock, INFINITE);
} }
/* It's my turn now -- I hold the potato */ /* It's my turn now -- I hold the potato */
SignalObjectAndWait(lock->lock, cond->event, INFINITE, FALSE);
slock_lock(lock); /* I still have the main lock, in any case */
/* I need to release it so that someone can set the event */
ReleaseMutex(lock->lock);
/* Wait for someone to actually signal this condition */
/* We're the only waiter waiting on the event right now -- everyone else is waiting on something different */
WaitForSingleObject(cond->event, INFINITE);
/* Take the main lock so we can do work. Nobody else waits on this lock for very long, so even though it's GO TIME we won't have to wait long */
WaitForSingleObject(lock->lock, INFINITE);
/* Remove ourselves from the queue */ /* Remove ourselves from the queue */
cond->head = myentry.next; cond->head = myentry.next;
@ -451,7 +473,7 @@ void scond_wait(scond_t *cond, slock_t *lock)
/* If any other wakenings are pending, go ahead and set it up */ /* If any other wakenings are pending, go ahead and set it up */
/* There may actually be no waiters. That's OK. The first waiter will come in, find it's his turn, and immediately get the signaled event */ /* There may actually be no waiters. That's OK. The first waiter will come in, find it's his turn, and immediately get the signaled event */
cond->wakens--; cond->wakens--;
if(cond->wakens>0) if (cond->wakens > 0)
{ {
SetEvent(cond->event); SetEvent(cond->event);