Author: Michael Buch Date: 2023-05-05T11:40:23-04:00 New Revision: e74f03dde5d8973e7ca3f99ea0d8d14c048d168d
URL: https://github.com/llvm/llvm-project/commit/e74f03dde5d8973e7ca3f99ea0d8d14c048d168d DIFF: https://github.com/llvm/llvm-project/commit/e74f03dde5d8973e7ca3f99ea0d8d14c048d168d.diff LOG: [lldb][TypeSystem] ForEach: Don't hold the TypeSystemMap lock across callback The `TypeSystemMap::m_mutex` guards against concurrent modifications of members of `TypeSystemMap`. In particular, `m_map`. `TypeSystemMap::ForEach` iterates through the entire `m_map` calling a user-specified callback for each entry. This is all done while `m_mutex` is locked. However, there's nothing that guarantees that the callback itself won't call back into `TypeSystemMap` APIs on the same thread. This lead to double-locking `m_mutex`, which is undefined behaviour. We've seen this cause a deadlock in the swift plugin with following backtrace: ``` int main() { std::unique_ptr<int> up = std::make_unique<int>(5); volatile int val = *up; return val; } clang++ -std=c++2a -g -O1 main.cpp ./bin/lldb -o “br se -p return” -o run -o “v *up” -o “expr *up” -b ``` ``` frame #4: std::lock_guard<std::mutex>::lock_guard frame #5: lldb_private::TypeSystemMap::GetTypeSystemForLanguage <<<< Lock #2 frame #6: lldb_private::TypeSystemMap::GetTypeSystemForLanguage frame #7: lldb_private::Target::GetScratchTypeSystemForLanguage ... frame #26: lldb_private::SwiftASTContext::LoadLibraryUsingPaths frame #27: lldb_private::SwiftASTContext::LoadModule frame #30: swift::ModuleDecl::collectLinkLibraries frame #31: lldb_private::SwiftASTContext::LoadModule frame #34: lldb_private::SwiftASTContext::GetCompileUnitImportsImpl frame #35: lldb_private::SwiftASTContext::PerformCompileUnitImports frame #36: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetSwiftASTContext frame #37: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetPersistentExpressionState frame #38: lldb_private::Target::GetPersistentSymbol frame #41: lldb_private::TypeSystemMap::ForEach <<<< Lock #1 frame #42: lldb_private::Target::GetPersistentSymbol frame #43: lldb_private::IRExecutionUnit::FindInUserDefinedSymbols frame #44: lldb_private::IRExecutionUnit::FindSymbol frame #45: lldb_private::IRExecutionUnit::MemoryManager::GetSymbolAddressAndPresence frame #46: lldb_private::IRExecutionUnit::MemoryManager::findSymbol frame #47: non-virtual thunk to lldb_private::IRExecutionUnit::MemoryManager::findSymbol frame #48: llvm::LinkingSymbolResolver::findSymbol frame #49: llvm::LegacyJITSymbolResolver::lookup frame #50: llvm::RuntimeDyldImpl::resolveExternalSymbols frame #51: llvm::RuntimeDyldImpl::resolveRelocations frame #52: llvm::MCJIT::finalizeLoadedModules frame #53: llvm::MCJIT::finalizeObject frame #54: lldb_private::IRExecutionUnit::ReportAllocations frame #55: lldb_private::IRExecutionUnit::GetRunnableInfo frame #56: lldb_private::ClangExpressionParser::PrepareForExecution frame #57: lldb_private::ClangUserExpression::TryParse frame #58: lldb_private::ClangUserExpression::Parse ``` Our solution is to simply iterate over a local copy of `m_map`. **Testing** * Confirmed on manual reproducer (would reproduce 100% of the time before the patch) Differential Revision: https://reviews.llvm.org/D149949 Added: Modified: lldb/source/Symbol/TypeSystem.cpp Removed: ################################################################################ diff --git a/lldb/source/Symbol/TypeSystem.cpp b/lldb/source/Symbol/TypeSystem.cpp index 9647102e96c61..24f2029305650 100644 --- a/lldb/source/Symbol/TypeSystem.cpp +++ b/lldb/source/Symbol/TypeSystem.cpp @@ -217,11 +217,21 @@ void TypeSystemMap::Clear() { void TypeSystemMap::ForEach( std::function<bool(lldb::TypeSystemSP)> const &callback) { - std::lock_guard<std::mutex> guard(m_mutex); + + // The callback may call into this function again causing + // us to lock m_mutex twice if we held it across the callback. + // Since we just care about guarding access to 'm_map', make + // a local copy and iterate over that instead. + collection map_snapshot; + { + std::lock_guard<std::mutex> guard(m_mutex); + map_snapshot = m_map; + } + // Use a std::set so we only call the callback once for each unique // TypeSystem instance. llvm::DenseSet<TypeSystem *> visited; - for (auto &pair : m_map) { + for (auto &pair : map_snapshot) { TypeSystem *type_system = pair.second.get(); if (!type_system || visited.count(type_system)) continue; _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits
