[ThinLTO] Fix ThinLTO crash while destroying context

[ncharlie]

Summary

[ThinLTO] Fix ThinLTO crash while destroying context

Fix for PR32763

An assert that checks if a Ref was untracked fails during ThinLTO context cleanup. The issue is because lazy loading temporary nodes didn't properly track ValueAsMetadata nodes. This patch ensures that the temporary nodes are properly tracked when they're replaced with the value.

[davide]

Thanks for your first contribution, great GSoC start!
One note: this manifests with ThinLTO, but it might be a more generic metadata issue. Are you able to reproduce without ThinLTO (and maybe with a single file?)
If not, can you explain why?

[ncharlie]

In D33513#763708, @davide wrote:

Thanks for your first contribution, great GSoC start!

Thanks! This was a good chance to look closely at the code to start learning how it works.

One note: this manifests with ThinLTO, but it might be a more generic metadata issue. Are you able to reproduce without ThinLTO (and maybe with a single file?)

The bug only happens when lazy loading Metadata. Lazy loading occurs when module level metadata is being imported by ThinLTO (see https://reviews.llvm.org/diffusion/L/browse/llvm/trunk/lib/Bitcode/Reader/MetadataLoader.cpp;303885$781). I believe this is the only path that leads to lazy loading, so I don't think it can happen elsewhere.

[dexonsmith]

I'm probably a good person to review this, but it's been a while since I've thought this so I might need help to refresh my context. This use-tracking code is unfortunately rather subtle.

Also, the testcase looks like it could possibly be reduced...

include/llvm/IR/Metadata.h

716–723

Notice that in MDOperand, MetadataTracking is sometimes called with an Owner argument.

This seems to only be used when MDNode::setOperand is called on a *uniqued* metadata node, or during MDNode initialization, when MDNode::makeUniqued() is called.

Since these operands are only for *distinct* metadata nodes, what you've done below looks correct... can you confirm you have the same understanding I do?

1275–1283

I wonder if it would be more or less clear to use:

MetadataTracking::untrack(this);
assert(!Use && "Somehow this is still being tracked...");

here, instead of just Use = nullptr.

test/Linker/Inputs/lazy-load-temporaries-cleanup.b.ll

5 ↗

(On Diff #100119)

Does this need to be !llvm.asan.globals, or can you make it !named and then prune the arguments to something minimal?

[mehdi_amini]

In D33513#764629, @ncharlie wrote:

In D33513#763708, @davide wrote:

Thanks for your first contribution, great GSoC start!

Thanks! This was a good chance to look closely at the code to start learning how it works.

One note: this manifests with ThinLTO, but it might be a more generic metadata issue. Are you able to reproduce without ThinLTO (and maybe with a single file?)

The bug only happens when lazy loading Metadata. Lazy loading occurs when module level metadata is being imported by ThinLTO (see https://reviews.llvm.org/diffusion/L/browse/llvm/trunk/lib/Bitcode/Reader/MetadataLoader.cpp;303885$781). I believe this is the only path that leads to lazy loading, so I don't think it can happen elsewhere.

If there is no other path to exercise LazyLoading of Metadata, let's just create one! (add an option to llvm-dis for example).

[dexonsmith]

In D33513#765222, @mehdi_amini wrote:

If there is no other path to exercise LazyLoading of Metadata, let's just create one! (add an option to llvm-dis for example).

I agree.

[dexonsmith]

In D33513#765223, @dexonsmith wrote:

In D33513#765222, @mehdi_amini wrote:

If there is no other path to exercise LazyLoading of Metadata, let's just create one! (add an option to llvm-dis for example).

I agree.

Or it could be an option to llvm-link (with a single file), where lazy-loading perhaps is more natural.

[ncharlie]

fixed tests

[ncharlie]

include/llvm/IR/Metadata.h

716–723

Notice that in MDOperand, MetadataTracking is sometimes called with an Owner argument.

Yep, I can confirm that the only places I've seen the Owner argument used is in MDNode::setOperand. It's also used in MetadataAsValue::track (though I don't think I've seen that happen in my test cases).

Since these operands are only for *distinct* metadata nodes, what you've done below looks correct... can you confirm you have the same understanding I do?

Yeah - this placeholder is only used for distinct nodes (the only place the they're created is when an attempt to get a distinct and resolved metadata fails: https://reviews.llvm.org/diffusion/L/browse/llvm/trunk/lib/Bitcode/Reader/MetadataLoader.cpp;303994$926)

1275–1283

Use = nullptr is more direct and shorter, but they both accomplish the same goal. I can have it call untrack instead if you want.

test/Linker/Inputs/lazy-load-temporaries-cleanup.b.ll

5 ↗

(On Diff #100119)

I can use !named but I don't think I can simplify the number of arguments any further, otherwise lazy loading isn't triggered.

[mehdi_amini]

test/Linker/Inputs/lazy-load-temporaries-cleanup.b.ll

5 ↗

(On Diff #100119)

There is an option in the bitcode reader/writer to disable the threshold of the minimum number of MDs to trigger lazy-loading.

[pcc]

test/ThinLTO/X86/lazy-load-temporaries-cleanup.ll

5 ↗

(On Diff #100419)

The llvm test suite cannot depend on lld.

Did you try llvm-dis -materialize-metadata?

[ncharlie]

Simplified test

[ncharlie]

@pcc @mehdi_amini I've simplified the tests using your recommendations. Note that to get lazy loading to work with llvm-dis, I had to set the IsImporting bool to true - I'm not sure if this is correct, but I couldn't set the value to true otherwise. Should I add a flag to allow enabling/disabling it instead of hardcoding it?

[dexonsmith]

The current patch seems to be missing context on Phab (-U999999). Please remember to add it on the next iteration.

include/llvm/IR/Metadata.h

1280

Nit: should be a space between if and (*Use). Running clang-format on your diff should clean this up. See the script at the bottom of the docs:
https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting

1282

Please at least add a comment saying that this is equivalent to MetadataTracking::untrack.

I do weakly prefer the untrack and assert combination, because it's easier to scan the code looking for balanced track/untrack calls. Also easier to add logging to track/untrack to sort out what's going on when there's a bug. I guess my point is it improves how clear the code is globally.

But it's a weak preference, and Use = nullptr is certainly more clear locally. What do others think?

tools/llvm-dis/llvm-dis.cpp

145–147

I haven't thought about whether this should be hardcoded (likely depends on whether we're losing important coverage in the false case -- have you looked at that?), but there should certainly be a /*What is this doing?=*/ comment.

[mehdi_amini]

test/ThinLTO/X86/lazy-load-temporaries-cleanup.ll

4 ↗

(On Diff #100620)

Just a nit: usually we don't split the RUN commands using a temporary file unless needed (i.e. use a pipe, opt can print on stdout and llvm-dis reads on stdin).

[ncharlie]

Added context

[ncharlie]

tools/llvm-dis/llvm-dis.cpp

145–147

Yup, just ran the test suite and a few tests will fail if I hard-code it. Should I just add a hidden flag to the llvm-dis utility then?

[ncharlie]

• Added hidden "set-importing" flag to llvm-dis to set the IsImporting boolean for testing.
• Ran clang-format
• Added comment
• Smaller test

[ncharlie]

Addressed comments. Formatted with clang-format, so that's why there are so many extra changes in the diff.

[mehdi_amini]

In D33513#772844, @ncharlie wrote:

Addressed comments. Formatted with clang-format, so that's why there are so many extra changes in the diff.

The common practice is that we never clang-format the full files. That's why LLVM has clang/tools/clang-format/git-clang-format ; if your commit is the last one in the history you can run git clang-format HEAD~ and it'll format only the statements touched by this last commit (you can also format changes that are about to be committed with git clang-format -f).

[davide]

In D33513#772857, @mehdi_amini wrote:

In D33513#772844, @ncharlie wrote:

Addressed comments. Formatted with clang-format, so that's why there are so many extra changes in the diff.

The common practice is that we never clang-format the full files. That's why LLVM has clang/tools/clang-format/git-clang-format ; if your commit is the last one in the history you can run git clang-format HEAD~ and it'll format only the statements touched by this last commit (you can also format changes that are about to be committed with git clang-format -f).

So, clang-formatt'ing the whole file is generally a good thing.
What I recommend is splitting that change in a separate NFC commit, and then reapply your patch on top of it.

[ncharlie]

Removed extraneous formatting.

[davide]

This one is getting there, one minor comment.

test/ThinLTO/X86/lazy-load-temporaries-cleanup.ll

3 ↗

(On Diff #102117)

one of the two files is empty, so maybe you can just inline?

Also, maybe dump the output of llvm-dis and check with filecheck?

[ncharlie]

Simplified test.

[ncharlie]

@dexonsmith @pcc any updates on this?

[ncharlie]

@dexonsmith @pcc ping?

[mehdi_amini]

include/llvm/IR/Metadata.h

1275–1283

My 2 cents: a call to untrack carries better the intent.

[mehdi_amini]

LGTM.

[davide]

LGTM

[ncharlie]

oops - forgot a cast

[ncharlie]

Committed: rL310967

include/llvm/IR/Metadata.h

     if (!Use)
       return;
     *Use = MD;
-    Use = nullptr;
+
+    if (*Use)
+      MetadataTracking::track(*Use);
+
+    Metadata *T = cast<Metadata>(this);
+    MetadataTracking::untrack(T);
+    assert(!Use && "Use is still being tracked despite being untracked!");
   }
 };
 

tools/llvm-dis/llvm-dis.cpp

 DontPrint("disable-output", cl::desc("Don't output the .ll file"), cl::Hidden);
 
 static cl::opt<bool>
-ShowAnnotations("show-annotations",
-                cl::desc("Add informational comments to the .ll file"));
+    SetImporting("set-importing",
+                 cl::desc("Set lazy loading to pretend to import a module"),
+                 cl::Hidden);
 
+static cl::opt<bool>
+    ShowAnnotations("show-annotations",
+                    cl::desc("Add informational comments to the .ll file"));
+
 static cl::opt<bool> PreserveAssemblyUseListOrder(
     "preserve-ll-uselistorder",
     cl::desc("Preserve use-list order when writing LLVM assembly."),

 static std::unique_ptr<Module> openInputFile(LLVMContext &Context) {
   std::unique_ptr<MemoryBuffer> MB =
       ExitOnErr(errorOrToExpected(MemoryBuffer::getFileOrSTDIN(InputFilename)));
-  std::unique_ptr<Module> M =
-      ExitOnErr(getOwningLazyBitcodeModule(std::move(MB), Context,
-                                           /*ShouldLazyLoadMetadata=*/true));
+  std::unique_ptr<Module> M = ExitOnErr(getOwningLazyBitcodeModule(
+      std::move(MB), Context,
+      /*ShouldLazyLoadMetadata=*/true, SetImporting));
   if (MaterializeMetadata)
     ExitOnErr(M->materializeMetadata());
   else