[Lldb-commits] [PATCH] D48393: Make DWARFParsing more thread-safe

Jonas Devlieghere via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Tue Sep 25 01:11:30 PDT 2018

JDevlieghere added a comment.

Thanks for the information, Greg!

In https://reviews.llvm.org/D48393#1243588, @clayborg wrote:

> A little background might help here: The lldb_private::Module lock is used to prevent multiple queries into the DWARF from stomping on each other.
> Multi-threaded DWARF parsing was primarily added to speed up indexing and the only place it is used. Is that not true? Indexing is just creating name tables and the accelerator tables we need so that we can partially parse the DWARF later. No type parsing should be happening when indexing. All other accesses to the DWARF must be done via a SymbolFile API that takes the module lock to stop multiple threads from stomping on each other. So my main point is we need to use the module lock to avoid having multiple threads doing important work that will cause crashes.

Looking at `SymbolFileDWARF`, we only have two methods that take the module lock: `PreloadSymbols` and `CompleteType`.

> The only multi-threaded access to DWARF currently should be in the indexing only and no important parsing stuff should be going on.

Surely there can be a lot more going on, like two debuggers executing an expression at the same time?

> If we discover places where multiple threads are making accesses, we just need to ensure they take the module lock and everything should just work.

So what you're saying is that we should add the module lock to every endpoint in `SymbolFileDWARF` that can potentially modify our internal data structures (i.e. parsing)?


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