From a842f979c219ef245b09c4607aacc36fbca3f66b Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Tue, 19 Mar 2013 19:44:44 -0400
Subject: Simplify future implementation.

A read-write lock was a bad idea, as most consumers are probably
writers, not readers.
---
 libdimension/future-impl.h |  7 +++----
 libdimension/future.c      | 52 ++++++++++++++++++++++------------------------
 2 files changed, 28 insertions(+), 31 deletions(-)

diff --git a/libdimension/future-impl.h b/libdimension/future-impl.h
index c25a7c2..953d5a4 100644
--- a/libdimension/future-impl.h
+++ b/libdimension/future-impl.h
@@ -1,5 +1,5 @@
 /*************************************************************************
- * Copyright (C) 2010-2011 Tavian Barnes <tavianator@tavianator.com>     *
+ * Copyright (C) 2010-2013 Tavian Barnes <tavianator@tavianator.com>     *
  *                                                                       *
  * This file is part of The Dimension Library.                           *
  *                                                                       *
@@ -42,12 +42,11 @@ struct dmnsn_future {
   /** The worker thread. */
   pthread_t thread;
 
-  /** Read-write synchronization. */
-  pthread_rwlock_t *rwlock;
+  /** Mutex to guard progress and total. */
+  pthread_mutex_t *mutex;
 
   /** Condition variable for waiting for a particular amount of progress. */
   pthread_cond_t  *cond;
-  pthread_mutex_t *mutex;
 
   /** Minimum waited-on value. */
   double *min_wait;
diff --git a/libdimension/future.c b/libdimension/future.c
index c328815..f645944 100644
--- a/libdimension/future.c
+++ b/libdimension/future.c
@@ -1,5 +1,5 @@
 /*************************************************************************
- * Copyright (C) 2009-2011 Tavian Barnes <tavianator@tavianator.com>     *
+ * Copyright (C) 2009-2013 Tavian Barnes <tavianator@tavianator.com>     *
  *                                                                       *
  * This file is part of The Dimension Library.                           *
  *                                                                       *
@@ -34,17 +34,12 @@ dmnsn_new_future(void)
   future->progress = 0;
   future->total    = 1;
 
-  /* Initialize the rwlock, condition variable, and mutex */
-
-  future->rwlock = dmnsn_malloc(sizeof(pthread_rwlock_t));
-  dmnsn_initialize_rwlock(future->rwlock);
+  future->mutex = dmnsn_malloc(sizeof(pthread_mutex_t));
+  dmnsn_initialize_mutex(future->mutex);
 
   future->cond = dmnsn_malloc(sizeof(pthread_cond_t));
   dmnsn_initialize_cond(future->cond);
 
-  future->mutex = dmnsn_malloc(sizeof(pthread_mutex_t));
-  dmnsn_initialize_mutex(future->mutex);
-
   future->min_wait = dmnsn_malloc(sizeof(double));
   *future->min_wait = 1.0;
 
@@ -70,14 +65,11 @@ dmnsn_future_join(dmnsn_future *future)
 
     dmnsn_free(future->min_wait);
 
-    dmnsn_destroy_mutex(future->mutex);
-    dmnsn_free(future->mutex);
-
     dmnsn_destroy_cond(future->cond);
     dmnsn_free(future->cond);
 
-    dmnsn_destroy_rwlock(future->rwlock);
-    dmnsn_free(future->rwlock);
+    dmnsn_destroy_mutex(future->mutex);
+    dmnsn_free(future->mutex);
 
     dmnsn_free(future);
   }
@@ -92,15 +84,26 @@ dmnsn_future_cancel(dmnsn_future *future)
   pthread_cancel(future->thread);
 }
 
+/**
+ * Get the current progress, without locking anything.
+ *
+ * future->mutex must be locked for this call to be safe.
+ */
+static inline double
+dmnsn_future_progress_unlocked(const dmnsn_future *future)
+{
+  return (double)future->progress/future->total;
+}
+
 /* Get the current progress of the worker thread, in [0.0, 1.0] */
 double
 dmnsn_future_progress(const dmnsn_future *future)
 {
   double progress;
 
-  dmnsn_read_lock(future->rwlock);
-    progress = (double)future->progress/future->total;
-  dmnsn_unlock_rwlock(future->rwlock);
+  dmnsn_lock_mutex(future->mutex);
+    progress = dmnsn_future_progress_unlocked(future);
+  dmnsn_unlock_mutex(future->mutex);
 
   return progress;
 }
@@ -110,7 +113,7 @@ void
 dmnsn_future_wait(const dmnsn_future *future, double progress)
 {
   dmnsn_lock_mutex(future->mutex);
-    while (dmnsn_future_progress(future) < progress) {
+    while (dmnsn_future_progress_unlocked(future) < progress) {
       /* Set the minimum waited-on value */
       if (progress < *future->min_wait)
         *future->min_wait = progress;
@@ -124,9 +127,9 @@ dmnsn_future_wait(const dmnsn_future *future, double progress)
 void
 dmnsn_future_set_total(dmnsn_future *future, size_t total)
 {
-  dmnsn_write_lock(future->rwlock);
+  dmnsn_lock_mutex(future->mutex);
     future->total = total;
-  dmnsn_unlock_rwlock(future->rwlock);
+  dmnsn_unlock_mutex(future->mutex);
 }
 
 /* Increment the number of completed loop iterations */
@@ -138,12 +141,10 @@ dmnsn_future_increment(dmnsn_future *future)
      on cancellation */
   pthread_testcancel();
 
-  dmnsn_write_lock(future->rwlock);
+  dmnsn_lock_mutex(future->mutex);
     ++future->progress;
-  dmnsn_unlock_rwlock(future->rwlock);
 
-  dmnsn_lock_mutex(future->mutex);
-    if (dmnsn_future_progress(future) >= *future->min_wait) {
+    if (dmnsn_future_progress_unlocked(future) >= *future->min_wait) {
       *future->min_wait = 1.0;
       dmnsn_cond_broadcast(future->cond);
     }
@@ -154,11 +155,8 @@ dmnsn_future_increment(dmnsn_future *future)
 void
 dmnsn_future_done(dmnsn_future *future)
 {
-  dmnsn_write_lock(future->rwlock);
-    future->progress = future->total;
-  dmnsn_unlock_rwlock(future->rwlock);
-
   dmnsn_lock_mutex(future->mutex);
+    future->progress = future->total;
     dmnsn_cond_broadcast(future->cond);
   dmnsn_unlock_mutex(future->mutex);
 }
-- 
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