Michael137 created this revision.
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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)
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D149949
Files:
lldb/source/Symbol/TypeSystem.cpp
Index: lldb/source/Symbol/TypeSystem.cpp
===================================================================
--- lldb/source/Symbol/TypeSystem.cpp
+++ lldb/source/Symbol/TypeSystem.cpp
@@ -217,7 +217,17 @@
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;
Index: lldb/source/Symbol/TypeSystem.cpp
===================================================================
--- lldb/source/Symbol/TypeSystem.cpp
+++ lldb/source/Symbol/TypeSystem.cpp
@@ -217,7 +217,17 @@
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;
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