[lldb-dev] Reporting bugs which only affect (semi-proprietary) downstream consumers.

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Tue Jun 22 10:34:00 PDT 2021

> On Jun 22, 2021, at 10:10 AM, Adam HARRIES via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Hi all, 
> I've recently taken over maintenance of my company's llvm+lldb branch, where we have added support for our in-house architecture (in llvm) as well as support for debugging through both hardware and our simulator. Our llvm fork is public/open source, however many of our runtime libraries and drivers (which are linked into lldb, clang, etc, and provide built-ins and driver support etc) are not. 
> While attempting to update our branch from llvm-11 to llvm-12 we came across a commit[1] in lldb which quite reliably causes a deadlock when we launch a process to debug a core dump. Luckily, said commit simply modifies some concurrency primitives, and reverting it is sufficient to fix the bug without any further effects. We are quite confident that the commit is the issue, as we performed a thorough bisect which maintained "our" code unchanged throughout.
> Unfortunately, however, we are unable to reproduce this bug in any "open" architectures (such as x86-64, AArch64, etc), so are not entirely sure how we should go about reporting the bug. Additionally, it makes it difficult to open a discussion regarding whether the commit is correct (and thus we may need to modify our additions to lldb to match new implicit behaviour), as third parties may be unable to reproduce the issue. Finally, as the bug results in a deadlock (which requires a sigkill to end) we won't (as I understand it) be able to use a "Reproducer" to demonstrate the bug to third parties. 
> Although we are able to "solve" the issue locally (by reverting the commit), we feel that the better solution would be to feed back our findings to the community and solve the issue, rather than (privately) sweeping it under the rug. As components of our compiler are proprietary, however, this process becomes difficult due to the reasons listed above.
> To summarise, there are two main questions that I feel unable to answer: 
> - Is there an existing process for reporting bugs that only affect third parties, and which cannot be reproduced in "core" targets. 

I don't believe there is a formal process for this. Though I would suggest just submitting a bug and attaching stack traces of your deadlock. Loading a core file is very similar across all targets, so I can't imagine this being hard to reproduce with another core file? Is there something special about your core file or setup? I know that logging used to be able to cause deadlocks due to the Module::GetDescription(...) that tried to take the module lock. It no longer does this on top of tree. 
> - To what extend is it possible to discuss (or report) bugs "on faith" - as in without any concrete evidence that a third party can reproduce.
> We are currently looking into opening up our build process so that we are able to distribute binary libraries to enable third parties to build our compiler + debugger, but as this is currently a work-in-progress it is unfortunately not a solution to this issue. 
> Many thanks in advance for any and all advice.
> Yours,

I would go ahead and debug the deadlock, attach repro steps for how you are loading your core file (exact commands or APIs that are being used) and then maybe attach the output "bt all" so we can see all of the threads and see what is deadlocking your LLDB.


> -- 
> Adam Brouwers-Harries
> Compiler Engineer
> aharries at upmem.com <mailto:aharries at upmem.com>
> [1] Please note, I have specifically not named this commit as I wish to better understand the "meta"-bug filing process, and I do not wish to publicly assign blame for any bugs without understanding how and why I can do so respectfully and properly.
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