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

[Jorge Gorbe Moya via lldb-dev]

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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