From 138c793b0a58577713d98c98bb140cf1d9c79bee Mon Sep 17 00:00:00 2001
From: Douglas Rumbaugh <dbr4@psu.edu>
Date: Wed, 17 Jan 2024 18:22:00 -0500
Subject: Multiple concurrency bug fixes

A poorly organized commit with fixes for a variety of bugs that were
causing missing records. The core problems all appear to be fixed,
though there is an outstanding problem with tombstones not being
completely canceled. A very small number are appearing in the wrong
order during the static structure test.
---
 include/framework/structure/MutableBuffer.h | 58 ++++++++++++++++++-----------
 1 file changed, 37 insertions(+), 21 deletions(-)

(limited to 'include/framework/structure/MutableBuffer.h')

diff --git a/include/framework/structure/MutableBuffer.h b/include/framework/structure/MutableBuffer.h
index eeb3dc9..7edde2f 100644
--- a/include/framework/structure/MutableBuffer.h
+++ b/include/framework/structure/MutableBuffer.h
@@ -133,18 +133,18 @@ public:
         return m_tombstone_filter->get_memory_usage();
     }
 
-    BufferView<R> get_buffer_view() {
-        size_t head = get_head();
+    BufferView<R> get_buffer_view(size_t target_head) {
+        size_t head = get_head(target_head);
         auto f = std::bind(release_head_reference, (void *) this, head);
 
         return BufferView<R>(m_data, m_cap, head, m_tail.load(), m_tscnt.load(), m_tombstone_filter, f);
     }
 
-    BufferView<R> get_flush_buffer_view() {
-        size_t head = get_head();
+    BufferView<R> get_buffer_view() {
+        size_t head = get_head(m_head.load().head_idx);
         auto f = std::bind(release_head_reference, (void *) this, head);
 
-        return BufferView<R>(m_data, m_cap, head, head + m_lwm, m_tscnt.load(), m_tombstone_filter, f);
+        return BufferView<R>(m_data, m_cap, head, m_tail.load(), m_tscnt.load(), m_tombstone_filter, f);
     }
 
     /*
@@ -167,23 +167,39 @@ public:
         buffer_head new_hd = {new_head, 0};
         buffer_head cur_hd;
 
-        /* move the current head into the old head */
+        /* replace current head with new head */
         do {
-            buffer_head cur_hd = m_head.load();
-            m_old_head.store(cur_hd);
+            cur_hd = m_head.load();
         } while(!m_head.compare_exchange_strong(cur_hd, new_hd));
 
+        /* move the current head into the old head */
+        m_old_head.store(cur_hd);
+
         m_active_head_advance.store(false);
         return true;
     }
 
-    size_t get_head() {
+    /*
+     * FIXME: If target_head does not match *either* the old_head or the
+     * current_head, this routine will loop infinitely.
+     */
+    size_t get_head(size_t target_head) {
         buffer_head cur_hd, new_hd;
+        bool head_acquired = false;
 
         do {
-            cur_hd = m_head.load();
-            new_hd = {cur_hd.head_idx, cur_hd.refcnt + 1};
-        } while(!m_head.compare_exchange_strong(cur_hd, new_hd));
+            if (m_old_head.load().head_idx == target_head) {
+                cur_hd = m_old_head.load();
+                cur_hd.head_idx = target_head;
+                new_hd = {cur_hd.head_idx, cur_hd.refcnt + 1};
+                head_acquired = m_old_head.compare_exchange_strong(cur_hd, new_hd);
+            } else if (m_head.load().head_idx == target_head){
+                cur_hd = m_head.load();
+                cur_hd.head_idx = target_head;
+                new_hd = {cur_hd.head_idx, cur_hd.refcnt + 1};
+                head_acquired = m_head.compare_exchange_strong(cur_hd, new_hd);
+            }
+        } while(!head_acquired);
 
         return new_hd.head_idx;
     }
@@ -254,22 +270,22 @@ private:
 
         buffer_head cur_hd, new_hd;
         do {
-            if (buffer->m_head.load().head_idx == head) {
-                cur_hd = buffer->m_head;
-                assert(cur_hd.refcnt > 0);
+            if (buffer->m_old_head.load().head_idx == head) {
+                cur_hd = buffer->m_old_head;
+                if (cur_hd.refcnt == 0) continue;
                 new_hd = {cur_hd.head_idx, cur_hd.refcnt-1};
-
-                if (buffer->m_head.compare_exchange_strong(cur_hd, new_hd)) {
+                if (buffer->m_old_head.compare_exchange_strong(cur_hd, new_hd)) {
                     break;
                 }
             } else {
-                cur_hd = buffer->m_old_head;
-                assert(cur_hd.refcnt > 0);
+                cur_hd = buffer->m_head;
+                if (cur_hd.refcnt == 0) continue;
                 new_hd = {cur_hd.head_idx, cur_hd.refcnt-1};
-                if (buffer->m_old_head.compare_exchange_strong(cur_hd, new_hd)) {
+
+                if (buffer->m_head.compare_exchange_strong(cur_hd, new_hd)) {
                     break;
                 }
-            }
+            } 
             _mm_pause();
         } while(true);
     }
-- 
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