[lldb-dev] Help fixing deadlock in DWARF symbol preloading

Pavel Labath via lldb-dev lldb-dev at lists.llvm.org
Thu Feb 4 12:16:43 PST 2021

Please have a look at 
which is the last time this came up.

One quick'n'dirty solution would be to have `Module::ReportError` _try_ 
to get the module lock, and if it fails, just bail out. That obviously 
means you won't get to see the error message which triggerred the 
deadlock (though we could also play around with that and try printing 
the error message without the module description or something), but it 
will at least get you past that point...


On 04/02/2021 21:04, Jorge Gorbe Moya via lldb-dev wrote:
> Hi,
> I've found a deadlock in lldb (see attached test case, you can build it 
> with just `clang -o test test.s`), but I'm a total newbie and I have no 
> idea what's the right way to fix it.
> The problem happens when an error is found during DIE extraction when 
> preloading symbols. As far as I can tell, it goes like this:
> 1. Module::PreloadSymbols locks Module::m_mutex
> 2. A few layers below it, we end up in ManualDWARFIndex::Index, which 
> dispatches DIE extractions to a thread pool:
> |for (size_t i = 0; i < units_to_index.size(); ++i) 
> pool.async(extract_fn, i); pool.wait(); |
> 3. extract_fn in the snippet above ends up executing 
> DWARFDebugInfoEntry::Extract and when there's an error during 
> extraction, Module::GetDescription is called while generating the error 
> message.
> 4. Module::GetDescription tries to acquire Module::m_mutex from a 
> different thread, while the main thread has the mutex already locked and 
> it's waiting for DIE extraction to end, causing a deadlock.
> If we make Module::GetDescription not lock the problem disappears, so 
> the diagnosis looks correct, but I don't know what would be the right 
> way to fix it. Module::GetDescription looks more or less safe to call 
> without locking: it just prints m_arch, m_file, and m_object_name to a 
> string, and those look like fields that wouldn't change after the Module 
> is initialized, so maybe it's okay? But I feel like there must be a 
> better solution anyway. Any advice?
> Best,
> Jorge
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