[Lldb-commits] [lldb] Convert the StackFrameList mutex to a shared mutex. (PR #117252)

[Pavel Labath via lldb-commits]

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@@ -578,58 +607,36 @@ void StackFrameList::Dump(Stream *s) {
 
 StackFrameSP StackFrameList::GetFrameAtIndex(uint32_t idx) {
   StackFrameSP frame_sp;
-  std::lock_guard<std::recursive_mutex> guard(m_mutex);
   uint32_t original_idx = idx;
 
-  uint32_t inlined_depth = GetCurrentInlinedDepth();
-  if (inlined_depth != UINT32_MAX)
-    idx += inlined_depth;
+  // We're going to consult the m_frames.size, but if there are already
+  // enough frames for our request we don't want to block other readers, so
+  // first acquire the shared lock:
+  { // Scope for shared lock:
+    std::shared_lock<std::shared_mutex> guard(m_list_mutex);
+
+    uint32_t inlined_depth = GetCurrentInlinedDepth();
+    if (inlined_depth != UINT32_MAX)
+      idx += inlined_depth;
 
-  if (idx < m_frames.size())
-    frame_sp = m_frames[idx];
+    if (idx < m_frames.size())
+      frame_sp = m_frames[idx];
 
-  if (frame_sp)
-    return frame_sp;
+    if (frame_sp)
+      return frame_sp;
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labath wrote:

(optional, idea) If `GetFramesUpTo` somehow returned the frame at the given index then there wouldn't be a need for this block here. I'm thinking about this in conjunction with my other suggestion to have a read-locked fastpath in `GetFramesUpTo`.

https://github.com/llvm/llvm-project/pull/117252