[libunwind] add hexagon support #55
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May 14, 2023
…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 rust-lang#4: std::lock_guard<std::mutex>::lock_guard
frame rust-lang#5: lldb_private::TypeSystemMap::GetTypeSystemForLanguage <<<< Lock rust-lang#2
frame rust-lang#6: lldb_private::TypeSystemMap::GetTypeSystemForLanguage
frame rust-lang#7: lldb_private::Target::GetScratchTypeSystemForLanguage
...
frame rust-lang#26: lldb_private::SwiftASTContext::LoadLibraryUsingPaths
frame rust-lang#27: lldb_private::SwiftASTContext::LoadModule
frame rust-lang#30: swift::ModuleDecl::collectLinkLibraries
frame rust-lang#31: lldb_private::SwiftASTContext::LoadModule
frame rust-lang#34: lldb_private::SwiftASTContext::GetCompileUnitImportsImpl
frame rust-lang#35: lldb_private::SwiftASTContext::PerformCompileUnitImports
frame rust-lang#36: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetSwiftASTContext
frame rust-lang#37: lldb_private::TypeSystemSwiftTypeRefForExpressions::GetPersistentExpressionState
frame rust-lang#38: lldb_private::Target::GetPersistentSymbol
frame rust-lang#41: lldb_private::TypeSystemMap::ForEach <<<< Lock #1
frame rust-lang#42: lldb_private::Target::GetPersistentSymbol
frame rust-lang#43: lldb_private::IRExecutionUnit::FindInUserDefinedSymbols
frame rust-lang#44: lldb_private::IRExecutionUnit::FindSymbol
frame rust-lang#45: lldb_private::IRExecutionUnit::MemoryManager::GetSymbolAddressAndPresence
frame rust-lang#46: lldb_private::IRExecutionUnit::MemoryManager::findSymbol
frame rust-lang#47: non-virtual thunk to lldb_private::IRExecutionUnit::MemoryManager::findSymbol
frame rust-lang#48: llvm::LinkingSymbolResolver::findSymbol
frame rust-lang#49: llvm::LegacyJITSymbolResolver::lookup
frame rust-lang#50: llvm::RuntimeDyldImpl::resolveExternalSymbols
frame rust-lang#51: llvm::RuntimeDyldImpl::resolveRelocations
frame rust-lang#52: llvm::MCJIT::finalizeLoadedModules
frame rust-lang#53: llvm::MCJIT::finalizeObject
frame rust-lang#54: lldb_private::IRExecutionUnit::ReportAllocations
frame rust-lang#55: lldb_private::IRExecutionUnit::GetRunnableInfo
frame rust-lang#56: lldb_private::ClangExpressionParser::PrepareForExecution
frame rust-lang#57: lldb_private::ClangUserExpression::TryParse
frame rust-lang#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
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