Replace GCX_COOP with EBR for EEHashTable bucket reclamation by AaronRobinsonMSFT · Pull Request #124822 · dotnet/runtime

AaronRobinsonMSFT · 2026-02-24T21:03:49Z

Replace cooperative GC mode transitions (GCX_COOP_NO_THREAD_BROKEN) with Epoch-Based Reclamation for safe deferred deletion of old EEHashTable bucket arrays during resize.

Changes

Add shared g_EbrCollector collector with initialization in ceemain.cpp and cleanup in finalizerthread.cpp
Add AllocateEEHashBuckets FreeEEHashBuckets helpers in eehash.cpp, following the same pattern as HashMap's AllocateBuckets FreeBuckets
Remove SyncClean::AddEEHashTable and associated cleanup infrastructure from syncclean.cpp/.hpp
Remove redundant MemoryBarrier() before VolatilePtr::Store() — release semantics already provide the ordering guarantee

Follow-up to #124307 (Replace HashMap COOP transitions with EBR). Specifically see #124307 (comment).

dotnet-policy-service · 2026-02-24T21:04:54Z

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Copilot

Pull request overview

This PR replaces the prior GC cooperative-mode based scheme for safely reclaiming obsolete EEHashTable bucket arrays with Epoch-Based Reclamation (EBR), aligning EEHashTable resizing behavior with the earlier HashMap EBR work.

Changes:

Add a dedicated global EBR collector (g_EEHashEbr) and initialize it during EE startup; trigger cleanup from the finalizer thread’s “extra work” loop.
Refactor EEHashTable bucket allocation/freeing into helpers and use EBR critical regions + deferred deletion during table growth instead of GCX_COOP_NO_THREAD_BROKEN + SyncClean.
Simplify SyncClean by removing the EEHashTable-specific cleanup list and related APIs.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/coreclr/vm/syncclean.hpp	Removes EEHashTable cleanup list API; leaves `SyncClean::CleanUp()` only.
src/coreclr/vm/syncclean.cpp	Removes EEHashTable cleanup list storage and freeing logic.
src/coreclr/vm/finalizerthread.cpp	Adds `g_EEHashEbr` cleanup request detection and processing in finalizer extra-work path.
src/coreclr/vm/eehash.inl	Replaces COOP transitions with EBR critical regions; defers old bucket array deletion via EBR during growth.
src/coreclr/vm/eehash.h	Declares EEHashTable bucket allocation/free helpers.
src/coreclr/vm/eehash.cpp	Implements `AllocateEEHashBuckets` / `FreeEEHashBuckets`.
src/coreclr/vm/ebr.h	Declares `extern EbrCollector g_EEHashEbr`.
src/coreclr/vm/ebr.cpp	Defines global `g_EEHashEbr`.
src/coreclr/vm/ceemain.cpp	Initializes `g_EEHashEbr` during EE startup.

src/coreclr/vm/eehash.inl

src/coreclr/vm/syncclean.hpp

Replace cooperative GC mode transitions (GCX_COOP_NO_THREAD_BROKEN) with Epoch-Based Reclamation for safe deferred deletion of old EEHashTable bucket arrays during resize. - Add dedicated g_EEHashEbr collector (init, cleanup, critical regions) - Add AllocateEEHashBuckets/FreeEEHashBuckets helpers in eehash.cpp - Remove SyncClean::AddEEHashTable and associated cleanup infrastructure - Remove redundant MemoryBarrier before VolatilePtr::Store (release semantics) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.

src/coreclr/vm/eehash.inl

src/coreclr/vm/ceemain.cpp

The per-thread EbrThreadData is a single thread_local with one m_pCollector field, so it can only be associated with one collector at a time. Having two collectors caused the same TLS node to be linked into both thread lists, corrupting epoch tracking and thread detach cleanup. Both HashMap and EEHashTable use generic byte-array reclamation, so a single shared collector is sufficient. Added an assertion in GetOrCreateThreadData to guard against future reintroduction of multiple collectors. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Replace CrstLeafLock usage in AsyncContinuationsManager with a dedicated CrstAsyncContinuations crst type that has proper lock ordering (AcquiredBefore LeafLock). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 1 comment.

src/coreclr/vm/eehash.inl

am11 · 2026-02-26T15:30:50Z

Can this be replaced too:

runtime/src/coreclr/vm/stubmgr.cpp

Line 636 in 7a5e8fc

GCX_COOP_NO_THREAD_BROKEN();

if so, after that we can delete GCX_COOP_NO_THREAD_BROKEN definition from src/coreclr/vm/util.hpp.

AaronRobinsonMSFT · 2026-02-26T19:24:18Z

Can this be replaced too:

runtime/src/coreclr/vm/stubmgr.cpp

Line 636 in 7a5e8fc

GCX_COOP_NO_THREAD_BROKEN();

if so, after that we can delete GCX_COOP_NO_THREAD_BROKEN definition from src/coreclr/vm/util.hpp.

Excellent observation, that is a great follow-up PR to verify this is still needed.

Remove the g_fEEStarted conditional that immediately freed old bucket arrays during startup. Other threads (finalizer, debugger, tracing) are created before g_fEEStarted is set, so the single-threaded assumption was incorrect. Now always queue old buckets via EBR deferred deletion and always enter EBR critical regions, matching the safety guarantees that HashMap provides for async mode. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/coreclr/inc/CrstTypes.def

Copilot

Pull request overview

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

src/coreclr/vm/eehash.inl

src/coreclr/vm/ceemain.cpp

jkotas

LGTM, assuming the tests are green

Copilot AI review requested due to automatic review settings February 24, 2026 21:03

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AaronRobinsonMSFT force-pushed the ebr-eehash branch from b4f6cb0 to c8e7d2b Compare February 24, 2026 21:30

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Add dedicated CrstAsyncContinuations lock type

a0a3d70

Replace CrstLeafLock usage in AsyncContinuationsManager with a dedicated CrstAsyncContinuations crst type that has proper lock ordering (AcquiredBefore LeafLock). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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AaronRobinsonMSFT marked this pull request as ready for review February 26, 2026 20:27

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jkotas approved these changes Feb 26, 2026

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Remove exposure of the EBR holder from dac builds.

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AaronRobinsonMSFT merged commit 17ca03d into dotnet:main Feb 27, 2026
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AaronRobinsonMSFT deleted the ebr-eehash branch February 27, 2026 01:48

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[main] Source code updates from dotnet/runtime dotnet/dotnet#5129

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AaronRobinsonMSFT commented Feb 24, 2026 •

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