Revert 251178 "Trace event micro-optimization: use finer grained..."

due to data races raised on the tsan v2 bot.

BUG=343802

> Trace event micro-optimization: use finer grained lock.
> 
> GetCategoryGroupEnabledInternal is a hotspot.
> It traverses an append-only list, so it's safe to use a finer-grained lock,
> and avoid it altogether for the "fast path", i.e., when the category is
> already known.
> 
> BUG=
> 
> Review URL: https://codereview.chromium.org/144423009

TBR=bulach@chromium.org

Review URL: https://codereview.chromium.org/163613006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251262 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 27 insertions, 39 deletions

diff --git a/base/debug/trace_event_impl.cc b/base/debug/trace_event_impl.cc
index 5665208..ce25c38 100644
--- a/base/debug/trace_event_impl.cc
+++ b/base/debug/trace_event_impl.cc
@@ -100,8 +100,7 @@ const int g_category_categories_exhausted = 2;
 const int g_category_metadata = 3;
 const int g_category_trace_event_overhead = 4;
 const int g_num_builtin_categories = 5;
-// Skip default categories.
-base::subtle::AtomicWord g_category_index = g_num_builtin_categories;
+int g_category_index = g_num_builtin_categories; // Skip default categories.
 
 // The name of the current thread. This is used to decide if the current
 // thread name has changed. We combine all the seen thread names into the
@@ -1260,51 +1259,40 @@ const unsigned char* TraceLog::GetCategoryGroupEnabledInternal(
     const char* category_group) {
   DCHECK(!strchr(category_group, '"')) <<
       "Category groups may not contain double quote";
-  // The g_category_groups is append only, avoid using a lock for the fast path.
-  int current_category_index = base::subtle::NoBarrier_Load(&g_category_index);
+  AutoLock lock(lock_);
 
+  unsigned char* category_group_enabled = NULL;
   // Search for pre-existing category group.
-  for (int i = 0; i < current_category_index; ++i) {
+  for (int i = 0; i < g_category_index; i++) {
     if (strcmp(g_category_groups[i], category_group) == 0) {
-      return &g_category_group_enabled[i];
+      category_group_enabled = &g_category_group_enabled[i];
+      break;
     }
   }
 
-  unsigned char* category_group_enabled = NULL;
-  // This is the slow path: the lock is not held in the case above, so more
-  // than one thread could have reached here trying to add the same category.
-  // Only hold to lock when actually appending a new category, and
-  // check the categories groups again.
-  AutoLock lock(lock_);
-  for (int i = 0; i < g_category_index; ++i) {
-    if (strcmp(g_category_groups[i], category_group) == 0) {
-      return &g_category_group_enabled[i];
+  if (!category_group_enabled) {
+    // Create a new category group
+    DCHECK(g_category_index < MAX_CATEGORY_GROUPS) <<
+        "must increase MAX_CATEGORY_GROUPS";
+    if (g_category_index < MAX_CATEGORY_GROUPS) {
+      int new_index = g_category_index++;
+      // Don't hold on to the category_group pointer, so that we can create
+      // category groups with strings not known at compile time (this is
+      // required by SetWatchEvent).
+      const char* new_group = strdup(category_group);
+      ANNOTATE_LEAKING_OBJECT_PTR(new_group);
+      g_category_groups[new_index] = new_group;
+      DCHECK(!g_category_group_enabled[new_index]);
+      // Note that if both included and excluded patterns in the
+      // CategoryFilter are empty, we exclude nothing,
+      // thereby enabling this category group.
+      UpdateCategoryGroupEnabledFlag(new_index);
+      category_group_enabled = &g_category_group_enabled[new_index];
+    } else {
+      category_group_enabled =
+          &g_category_group_enabled[g_category_categories_exhausted];
     }
   }
-
-  // Create a new category group.
-  DCHECK(g_category_index < MAX_CATEGORY_GROUPS) <<
-      "must increase MAX_CATEGORY_GROUPS";
-  if (g_category_index < MAX_CATEGORY_GROUPS) {
-    int new_index = base::subtle::Acquire_Load(&g_category_index);
-    // Don't hold on to the category_group pointer, so that we can create
-    // category groups with strings not known at compile time (this is
-    // required by SetWatchEvent).
-    const char* new_group = strdup(category_group);
-    ANNOTATE_LEAKING_OBJECT_PTR(new_group);
-    g_category_groups[new_index] = new_group;
-    DCHECK(!g_category_group_enabled[new_index]);
-    // Note that if both included and excluded patterns in the
-    // CategoryFilter are empty, we exclude nothing,
-    // thereby enabling this category group.
-    UpdateCategoryGroupEnabledFlag(new_index);
-    category_group_enabled = &g_category_group_enabled[new_index];
-    // Update the max index now.
-    base::subtle::Release_Store(&g_category_index, new_index + 1);
-  } else {
-    category_group_enabled =
-        &g_category_group_enabled[g_category_categories_exhausted];
-  }
   return category_group_enabled;
 }