[lldb-dev] LLDB deadlocks trying to unwind

[Félix Cloutier]

It's obviously the only thread, since frame #8 acquired the mutex first and frame #6 tries to acquire it again.

I changed it to a recursive mutex, and now it crashes on a stack overflow instead of deadlocking. Here's an excerpt of the stack, from the initially calling frame (StackFrameList::GetFramesUpTo):

#12543	UnwindMacOSXFrameBackchain::DoGetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp:59
#12544	lldb_private::Unwind::GetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/include/lldb/Target/Unwind.h:78
#12545	lldb_private::StackFrameList::GetFramesUpTo(unsigned int) at lldb/source/Target/StackFrameList.cpp:332
#12546	lldb_private::StackFrameList::GetFrameAtIndex(unsigned int) at lldb/source/Target/StackFrameList.cpp:521
#12547	lldb_private::Thread::GetStackFrameAtIndex(unsigned int) at lldb/include/lldb/Target/Thread.h:357
#12548	UnwindMacOSXFrameBackchain::DoGetFrameCount() at lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp:45
#12549	lldb_private::Unwind::GetFrameCount() at lldb/include/lldb/Target/Unwind.h:52
#12550	UnwindMacOSXFrameBackchain::DoGetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp:59
#12551	lldb_private::Unwind::GetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/include/lldb/Target/Unwind.h:78
#12552	lldb_private::StackFrameList::GetFramesUpTo(unsigned int) at lldb/source/Target/StackFrameList.cpp:332
#12553	lldb_private::StackFrameList::GetFrameAtIndex(unsigned int) at lldb/source/Target/StackFrameList.cpp:521
#12554	lldb_private::Thread::GetStackFrameAtIndex(unsigned int) at lldb/include/lldb/Target/Thread.h:357
#12555	UnwindMacOSXFrameBackchain::DoGetFrameCount() at lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp:45
#12556	lldb_private::Unwind::GetFrameCount() at lldb/include/lldb/Target/Unwind.h:52
#12557	UnwindMacOSXFrameBackchain::DoGetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp:59
#12558	lldb_private::Unwind::GetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/include/lldb/Target/Unwind.h:78
#12559	lldb_private::StackFrameList::GetFramesUpTo(unsigned int) at lldb/source/Target/StackFrameList.cpp:304


Le 2013-08-22 à 16:53:15, "Malea, Daniel" <daniel.malea at intel.com> a écrit :

> Hmm, based on the stack trace you provided, it seems that m_unwind_mutex should be initialized as a recursive mutex, just like Process::m_thread_mutex.
> 
> Can you try to change the initialization of m_unwind_mutex in Unwind.h to match that of Process:m_thread_mutex, and see if it helps? If not, then it means some other thread is holding that mutex and more investigation is required…
> 
> Cheers,
> Dan
> 
> From: Félix Cloutier <felixcca at yahoo.ca<mailto:felixcca at yahoo.ca>>
> Date: Thursday, 22 August, 2013 4:26 PM
> To: "lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>" <lldb-dev at cs.uiuc.edu<mailto:lldb-dev at cs.uiuc.edu>>
> Subject: [lldb-dev] LLDB deadlocks trying to unwind
> 
> LLDB, connected to a stub I'm building, is trying to produce a stack trace of my stopped program but deadlocks instead:
> 
> #0 __psynch_mutexwait ()
> #1 pthread_mutex_lock ()
> #2 lldb_private::Mutex::Lock() at lldb/source/Host/common/Mutex.cpp:281
> #3 lldb_private::Mutex::Locker::Lock(lldb_private::Mutex&) at lldb/source/Host/common/Mutex.cpp:152
> #4 lldb_private::Mutex::Locker::Locker(lldb_private::Mutex&) at lldb/source/Host/common/Mutex.cpp:112
> #5 lldb_private::Mutex::Locker::Locker(lldb_private::Mutex&) at lldb/source/Host/common/Mutex.cpp:113
> #6 lldb_private::Unwind::GetFrameCount() at lldb/include/lldb/Target/Unwind.h:51
> #7 UnwindMacOSXFrameBackchain::DoGetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/source/Plugins/Process/Utility/UnwindMacOSXFrameBackchain.cpp:59
> #8 lldb_private::Unwind::GetFrameInfoAtIndex(unsigned int, unsigned long long&, unsigned long long&) at lldb/include/lldb/Target/Unwind.h:78
> #9 lldb_private::StackFrameList::GetFramesUpTo(unsigned int) at lldb/source/Target/StackFrameList.cpp:304
> #10 lldb_private::StackFrameList::ResetCurrentInlinedDepth() at lldb/source/Target/StackFrameList.cpp:110
> #11 lldb_private::StackFrameList::CalculateCurrentInlinedDepth() at lldb/source/Target/StackFrameList.cpp:79
> #12 lldb_private::Thread::ShouldStop(lldb_private::Event*) at lldb/source/Target/Thread.cpp:752
> #13 lldb_private::ThreadList::ShouldStop(lldb_private::Event*) at lldb/source/Target/ThreadList.cpp:298
> #14 lldb_private::Process::ShouldBroadcastEvent(lldb_private::Event*) at lldb/source/Target/Process.cpp:3707
> #15 lldb_private::Process::HandlePrivateEvent(std::__1::shared_ptr<lldb_private::Event>&) at lldb/source/Target/Process.cpp:3936
> #16 lldb_private::Process::ConnectRemote(lldb_private::Stream*, char const*) at lldb/source/Target/Process.cpp:3257
> #17 CommandObjectProcessConnect::DoExecute(lldb_private::Args&, lldb_private::CommandReturnObject&) at lldb/source/Commands/CommandObjectProcess.cpp:1173
> #18 lldb_private::CommandObjectParsed::Execute(char const*, lldb_private::CommandReturnObject&) at lldb/source/Interpreter/CommandObject.cpp:1038
> #19 lldb_private::CommandInterpreter::HandleCommand(char const*, lldb_private::LazyBool, lldb_private::CommandReturnObject&, lldb_private::ExecutionContext*, bool, bool) at lldb/source/Interpreter/CommandInterpreter.cpp:1825
> #20 lldb_private::CommandObjectRegexCommand::DoExecute(char const*, lldb_private::CommandReturnObject&) at lldb/source/Interpreter/CommandObjectRegexCommand.cpp:89
> #21 lldb_private::CommandObjectRaw::Execute(char const*, lldb_private::CommandReturnObject&) at lldb/source/Interpreter/CommandObject.cpp:1064
> #22 lldb_private::CommandInterpreter::HandleCommand(char const*, lldb_private::LazyBool, lldb_private::CommandReturnObject&, lldb_private::ExecutionContext*, bool, bool) at lldb/source/Interpreter/CommandInterpreter.cpp:1825
> #23 lldb::SBCommandInterpreter::HandleCommand(char const*, lldb::SBCommandReturnObject&, bool) at lldb/source/API/SBCommandInterpreter.cpp:122
> #24 Driver::HandleIOEvent(lldb::SBEvent const&) at lldb/tools/driver/Driver.cpp:1083
> #25 Driver::MainLoop() at lldb/tools/driver/Driver.cpp:1556
> #26 main at lldb/tools/driver/Driver.cpp:1727
> #27 start ()
> 
> #8 locks m_unwind_mutex and #6 tries to do the same thing.
> 
> The code path is unconditional (calling GetFrameInfoAtIndex with a UnwindMacOSXFrameBackchain can only end in a deadlock, from what I can see), so I'm not sure how I can prevent this from happening.
> 
> Félix