JDK-8302320: AsyncGetCallTrace obtains too few frames in sanity test #12535
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src/hotspot/cpu/x86/frame_x86.cpp
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| // unextended sp must be within the stack | ||
| if (!thread->is_in_full_stack_checked(unextended_sp)) { |
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I'm not at all sure this relaxation is valid. What are you basing this on?
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The test fails without this relaxation and the relaxation has been used on PPC to solve a similar issue a few years back.
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The original assumption does not always hold anymore and should therefore relaxed.
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I think unextended_sp < sp is possible on x86 when interpreter entries generated by MethodHandles::generate_method_handle_interpreter_entry() are executed. They modify sp (here for example) but not sender_sp (InterpreterMacroAssembler ::_bcp_register).
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From frame_x86.hpp:
// The interpreter and adapters will extend the frame of the caller.
// Since oopMaps are based on the sp of the caller before extension
// we need to know that value. However in order to compute the address
// of the return address we need the real "raw" sp. By convention we
// use sp() to mean "raw" sp and unextended_sp() to mean the caller's
// original sp.
by that definition unextended_sp is always > sp. If something has violated that definition/convention then we may have bigger problems!
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If an ordinary interpreter entry is used for a call this means the callee will be interpreted
Maybe we're not talking about the same thing?
Method has a from_interpreted_entry and a from_compiled_entry, but which one is used depends on the caller, and which calling convention they are using. The callee can be either interpreted or compiled (and if there's a mismatch, we go through an adapter). The entry generated by generate_method_handle_interpreter_entry is essentially the from_interpreted_entry, AFAIU.
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The entry generated by generate_method_handle_interpreter_entry is essentially the from_interpreted_entry, AFAIU.
But it is also stored in Method::_i2i_entry which is used by c2i adapters.
(Maybe too) long version:
The interpreter has entries depending on the MethodKind. See AbstractInterpreter::entry_for_kind(). The entries are stored in AbstractInterpreter::_entry_table. In MethodHandlesAdapterGenerator::generate() the interpreter entries for method handle intrinsics MethodKinds are generated using the generator MethodHandles::generate_method_handle_interpreter_entry().
In Method::link_method() the entry points are set depending on the MethodKind. The interpreter entry (see above) for the method's MethodKind is looked up and stored in Method::_i2i_entry which is also used by c2i adapters.
So to me it looks as if the entry generated by generate_method_handle_interpreter_entry() can be reached coming from a compiled caller. The generator for ordinary methods is TemplateInterpreterGenerator::generate_normal_entry(). The entries it generates are surely reached by compiled callers. I might be missing something in the case of MH intrinsics but right now it looks to me as if the caller can be compiled too.
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(Maybe too) long version:
Not at all :) I don't know much about the interpreter, so this is pretty helpful.
Ok. I think the main point is that a MH linker method doesn't have use a c2i adapter, it instead has separate versions for interpreted and compiled callers altogether (except linkToNative which doesn't have an interpreter version atm)
Longer version:
I think there are 4 theoretical cases (are there more?):
- interpreted -> MH interpreter entry -> interpreted (the case we want to fix, I think)
- interpreted -> MH interpreter entry -> i2c -> compiled
- compiled -> c2i -> MH interpreter entry -> interpreted
- compiled -> c2i -> MH interpreter entry -> i2c -> compiled
So... my understanding is that a c2i adapter is used when a callee is interpreted, so its from_compiled_entry points to the c2i adapter. A compiled caller calls the from_compiled_entry, so it ends up going through the c2i adapter of an interpreted callee. However, for method handle linkers, the from_compiled_entry never points to a c2i adapter, but to the MH linker compiler entry generated in gen_special_dispatch in e.g sharedRuntime_x86_64. So, in other words, the 3. and 4. scenarios above are not possible, AFAICS. An MH linker's interpreted/compiled entries simply replace the regular i2c and c2i adapters for a MH linker method. Though, the actual target method can be either compiled or interpreted, so we can go into either a c2i adapter from a MH linker compiler entry, or a i2c adapter from a MH linker interpreter entry (which is scenario 2.).
For scenario 1.: caller sets r13 before jumping to callee (MH linker), we end up in the MH adapter, pop the last argument, and adjust r13 for the actual callee. The MH interpreter entry uses from_interpreted_entry of the target method, so we don't go through any adapter.
For scenario 2.: mostly the same as 1., but we go through an i2c adapter, and the compiled callee doesn't care about what's in r13 any ways. (there's a comment in the ic2 adapter code about preserving r13 for the case that we might go back into a c2i adapter if we lose a race where the callee is made non-entrant, but r13 is also used as a scratch register later in the i2c code, and the c2i code overwrites r13 any ways. So, I think that is an outdated comment...)
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So... my understanding is that a c2i adapter is used when a callee is
interpreted, so itsfrom_compiled_entrypoints to the c2i adapter. A
compiled caller calls thefrom_compiled_entry, so it ends up going through
thec2iadapter of an interpreted callee. However, for method handle
linkers, thefrom_compiled_entrynever points to a c2i adapter, but to the
MH linker compiler entry generated ingen_special_dispatchin e.g
sharedRuntime_x86_64. So, in other words, the 3. and 4. scenarios above are
not possible, AFAICS.
I see. Thanks! That's what I've been missing. It's implemented in SystemDictionary::find_method_handle_intrinsic() I think.
I agree that it will be ok to modify r13 as you suggested. Maybe explaining why we never reach here coming from a c2i adapter: because native wrappers are generated eagerly in SystemDictionary::find_method_handle_intrinsic() unless -Xint is given but then the caller cannot be compiled.
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I integrated your suggestion, it seems to work now. I'm hopeful that our internal queue does not find a test error.
I'm happy for any commenting suggestions.
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There are crashes in stack walking with this change. Try e.g. vmTestbase/vm/mlvm/indy/func/jvmti/stepBreakPopReturn/INDIFY_Test.java |
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There also errors on Linux, e.g. So back to my original suggestion which is simple and worked (ran it through our internal test queue and found no errors)? |
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Java stack |
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Thanks for trying it out! I ran tier 1-4 here as well, and all failing tests are in Also, I later realized that interpreter frames also save the sp before before making a call ( There's probably a bit of tech debt here (filed: https://bugs.openjdk.org/browse/JDK-8302745). I had a closer look at the original fix, and it looks like the |
Could you be so kind to synthesize a comment for me? I'm slightly overwhelmed by the discussion. |
Something like: |
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Thanks, I modified my PR accordingly. |
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Is this PR then ready? |
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I'm somewhat concerned that the code doesn't seem to have a clear notion of exactly how |
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The new check looks good to me. I think the only potential issue is in cases where unextended_sp == sp and sp - Interpreter::stackElementSize is somehow outside of the stack. But, since this is looking at a sender, it would mean that the current frame would definitely be outside of the stack, so I think we're good.
I'll submit tier 1-4 on our CI, and give a check mark when it comes back green.
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Going to push as commit db483a3.
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@JornVernee @parttimenerd Pushed as commit db483a3. 💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored. |
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/backport jdk17u |
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@parttimenerd To use the |
Extends the existing AsyncGetCallTrace test case and fixes the issue.
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