Please have a look at
<https://lists.llvm.org/pipermail/lldb-dev/2020-October/016479.html>,
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...
pl
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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