[Lldb-commits] [lldb] 9621bed - [lldb] Only preload a Module's symbols once (#213094)

[via lldb-commits]

Author: Charles Zablit
Date: 2026-07-31T11:49:30+02:00
New Revision: 9621bedd8a7a09c5e48a12697ff2d0463067e341

URL: https://github.com/llvm/llvm-project/commit/9621bedd8a7a09c5e48a12697ff2d0463067e341
DIFF: https://github.com/llvm/llvm-project/commit/9621bedd8a7a09c5e48a12697ff2d0463067e341.diff

LOG: [lldb] Only preload a Module's symbols once (#213094)

Creating several targets for the same file concurrently could deadlock.

Targets share Module objects. Therefore, each concurrent
`SBDebugger::CreateTarget()` call creates a `PreloadSymbols()` task for
the *same* Module. The thread pool running those tasks can pick up a
duplicate preload task for the same Module. The mutex used by the Module
is recursive, so the thread starts the task again, enters the same
`std::call_once` and deadlocks.

Preloading only needs to happen once: it does work taht a later lookup
would do anyway. If it's already in progress, skip it. This deduplicate
the task that causes the deadlock.

At desk, this fixes a timeout in
`api/multiple-targets/TestMultipleTargets.py`, roughly 1/40 runs. To
reproduce the issue, I shrunk the thread pool to 2 threads, which makes
it always timeout.

Added: 
    

Modified: 
    lldb/include/lldb/Core/Module.h
    lldb/source/Core/Module.cpp

Removed: 
    


################################################################################
diff  --git a/lldb/include/lldb/Core/Module.h b/lldb/include/lldb/Core/Module.h
index 33904ef7be5d8..1ca8e1843d02c 100644
--- a/lldb/include/lldb/Core/Module.h
+++ b/lldb/include/lldb/Core/Module.h
@@ -1122,6 +1122,7 @@ class Module : public std::enable_shared_from_this<Module>,
   std::atomic<bool> m_did_load_objfile{false};
   std::atomic<bool> m_did_load_symfile{false};
   std::atomic<bool> m_did_set_uuid{false};
+  std::atomic<bool> m_did_preload_symbols{false};
   mutable bool m_file_has_changed : 1,
       m_first_file_changed_log : 1; /// See if the module was modified after it
                                     /// was initially opened.

diff  --git a/lldb/source/Core/Module.cpp b/lldb/source/Core/Module.cpp
index dcdfd67d3e303..da7dec6d97bc2 100644
--- a/lldb/source/Core/Module.cpp
+++ b/lldb/source/Core/Module.cpp
@@ -1336,6 +1336,9 @@ void Module::FindSymbolsMatchingRegExAndType(
 }
 
 void Module::PreloadSymbols() {
+  if (m_did_preload_symbols.exchange(true))
+    return;
+
   LockedPtr<SymbolFile> sym_file = GetSymbolFileLocked();
   if (!sym_file)
     return;
@@ -1409,6 +1412,7 @@ void Module::SetSymbolFileFileSpec(const FileSpec &file) {
   m_symfile_spec = file;
   m_symfile_up.reset();
   m_did_load_symfile = false;
+  m_did_preload_symbols = false;
 }
 
 bool Module::IsExecutable() {