8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947 #8756

theRealAph · 2022-05-17T18:08:58Z

This is an assertion failure caused by setting BlockZeroingLowLimit < wordSize.

I believe there was some confusion when writing this code about whether BlockZeroingLowLimit should be in words or bytes, and it makes no sense for it to be less than a single word. If anyone ever tried to use a parameter < 8, it triggers an assertion.

This patch does two things. Firstly, it corrects a guarantee which erroneously used zero_words_block_size rather than wordSize. The value of both of these is 8, so it doesn't change anything in the generated code. Secondly, it clips the BlockZeroingLowLimit so that it doesn't trigger the assertion.

Perhaps it would be better to change the lower limit to 8 instead of this silent correction. There is no backward compatibility issue here, because any attempt to set BlockZeroingLowLimit < 8 in the past would have exited the VM with an error, so I don't believe a CSR is warranted if we do change the allowable range.

So, which should it be? Change the lower limit of the range of the BlockZeroingLowLimit system flag, or allow 1 still to be used and silently fix it? Opinions welcome.

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Issue

JDK-8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947

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Aleksey Shipilev (@shipilev - Reviewer)
Vladimir Kozlov (@vnkozlov - Reviewer) ⚠️ Review applies to f6984247

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bridgekeeper · 2022-05-17T18:10:13Z

👋 Welcome back aph! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2022-05-17T18:12:36Z

@theRealAph The following label will be automatically applied to this pull request:

hotspot

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mlbridge · 2022-05-17T18:16:14Z

Webrevs

vnkozlov · 2022-05-17T18:26:48Z

Change the lower limit of the range of the BlockZeroingLowLimit system flag.
Seems RISCV code has the same issue.

theRealAph · 2022-05-17T18:38:11Z

Change the lower limit of the range of the BlockZeroingLowLimit system flag. Seems RISCV code has the same issue.

OK.

shipilev · 2022-05-18T06:27:44Z

Why not just do:

diff --git a/src/hotspot/cpu/aarch64/globals_aarch64.hpp b/src/hotspot/cpu/aarch64/globals_aarch64.hpp
index 56fc873267d..b6310127a4d 100644
--- a/src/hotspot/cpu/aarch64/globals_aarch64.hpp
+++ b/src/hotspot/cpu/aarch64/globals_aarch64.hpp
@@ -105,7 +105,7 @@ define_pd_global(intx, InlineSmallCode,          1000);
           "Use DC ZVA for block zeroing")                               \
   product(intx, BlockZeroingLowLimit, 256,                              \
           "Minimum size in bytes when block zeroing will be used")      \
-          range(1, max_jint)                                            \
+          range(wordSize, max_jint)                                     \
   product(bool, TraceTraps, false, "Trace all traps the signal handler")\
   product(int, SoftwarePrefetchHintDistance, -1,                        \
           "Use prfm hint with specified distance in compiled code."     \

?

shipilev · 2022-05-19T16:51:14Z

src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp

@@ -4855,7 +4855,7 @@ address MacroAssembler::zero_words(Register ptr, Register cnt)
 // r10, r11, rscratch1, and rscratch2 are clobbered.
 address MacroAssembler::zero_words(Register base, uint64_t cnt)
 {
-  guarantee(zero_words_block_size < BlockZeroingLowLimit,
+  guarantee(wordSize <= BlockZeroingLowLimit,


Surely, this guarantee would now never fire? Maybe we should demote it to assert to protect from accidental range error. There is no need to pay the price in release bits, I think.

openjdk · 2022-05-19T16:52:39Z

@theRealAph This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947

Reviewed-by: shade, kvn

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 128 new commits pushed to the master branch:

f7a37f5: 8287202: GHA: Add macOS aarch64 to the list of default platforms for workflow_dispatch event
d889319: 8287254: Clean up Xcode sysroot logic
593d2b7: 8287091: aarch64 : guarantee(val < (1ULL << nbits)) failed: Field too big for insn
6585043: 8287181: Avoid redundant HashMap.containsKey calls in InternalLocaleBuilder.setExtension
a0cccb5: 8286490: JvmtiEventControllerPrivate::set_event_callbacks CLEARING_MASK computation is incorrect
742644e: 8287263: java/nio/channels/FileChannel/LargeMapTest.java times out on Windows
d792cbc: 8286211: Update PCSC-Lite for Suse Linux to 1.9.5
9b7e42c: 8284209: Replace remaining usages of 'a the' in source code
6cc4bb1: 6725221: Standardize obtaining boolean properties with defaults
194bc08: 8287249: Zero: Missing BarrierSetNMethod::arm() method
... and 118 more: https://git.openjdk.java.net/jdk/compare/c0d51d42d9715b44df995328bba978ba61dec3af...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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vnkozlov

Good.

shipilev

Still good.

theRealAph · 2022-05-25T09:02:09Z

/integrate

openjdk · 2022-05-25T09:03:23Z

Going to push as commit e17118a.
Since your change was applied there have been 128 commits pushed to the master branch:

f7a37f5: 8287202: GHA: Add macOS aarch64 to the list of default platforms for workflow_dispatch event
d889319: 8287254: Clean up Xcode sysroot logic
593d2b7: 8287091: aarch64 : guarantee(val < (1ULL << nbits)) failed: Field too big for insn
6585043: 8287181: Avoid redundant HashMap.containsKey calls in InternalLocaleBuilder.setExtension
a0cccb5: 8286490: JvmtiEventControllerPrivate::set_event_callbacks CLEARING_MASK computation is incorrect
742644e: 8287263: java/nio/channels/FileChannel/LargeMapTest.java times out on Windows
d792cbc: 8286211: Update PCSC-Lite for Suse Linux to 1.9.5
9b7e42c: 8284209: Replace remaining usages of 'a the' in source code
6cc4bb1: 6725221: Standardize obtaining boolean properties with defaults
194bc08: 8287249: Zero: Missing BarrierSetNMethod::arm() method
... and 118 more: https://git.openjdk.java.net/jdk/compare/c0d51d42d9715b44df995328bba978ba61dec3af...master

Your commit was automatically rebased without conflicts.

openjdk · 2022-05-25T09:03:38Z

@theRealAph Pushed as commit e17118a.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947

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openjdk bot added the rfr Pull request is ready for review label May 17, 2022

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openjdk bot removed the rfr Pull request is ready for review label May 17, 2022

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8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947 #8756

8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947 #8756

theRealAph commented May 17, 2022 •

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8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947 #8756

8272791: java -XX:BlockZeroingLowLimit=1 crashes after 8270947 #8756

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theRealAph commented May 17, 2022 • edited by openjdk bot

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