Don't use callbacks when performing sync IO with async handles by Copilot · Pull Request #126845 · dotnet/runtime

Copilot · 2026-04-13T17:28:11Z

Description

When performing synchronous I/O on async file handles (FILE_FLAG_OVERLAPPED), RandomAccess.Windows.cs previously used ThreadPoolBoundHandle.UnsafeAllocateNativeOverlapped with an IOCompletionCallback and a custom CallbackResetEvent class that tracked a reference count to handle the race between the IOCP callback and the caller's GetOverlappedResult.

This callback path is unnecessary. Setting the low-order bit of hEvent in the OVERLAPPED structure prevents the I/O completion from being queued to the completion port, eliminating the callback.

This reduces complexity and allocations, preserves managed wait behavior, fixes correctness issues in both error handling and exceptional wait paths, and hardens handle lifetime safety in the sync-over-async path.

…vent for sync IO with async handles Instead of using ThreadPoolBoundHandle callbacks and reference counting when performing synchronous IO with async file handles, allocate the NativeOverlapped directly via NativeMemory and set the low-order bit of hEvent to suppress I/O completion port notifications. This eliminates the CallbackResetEvent class, the s_callback field, and the AllocateCallback/GetNativeOverlappedForAsyncHandle methods, reducing allocations and complexity. Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/d7a61400-c63f-401a-9503-0313459af6f7 Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

dotnet-policy-service · 2026-04-13T20:25:05Z

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adamsitnik

@copilot please address my feedback (nits)

…appedWithEventHandle Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/bcdd8838-718e-4561-abf4-90c9c2deb905 Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

adamsitnik

The changes LGTM, the perf is much better despite still using ManualResetEvent:

(managed_cached is the PR)

BenchmarkDotNet v0.16.0-nightly.20260302.459, Windows 11 (10.0.26200.8390/25H2/2025Update/HudsonValley2)
AMD Ryzen Threadripper PRO 3945WX 12-Cores 3.99GHz, 1 CPU, 24 logical and 12 physical cores
Memory: 63.86 GB Total, 32.62 GB Available
.NET SDK 11.0.100-preview.5.26255.101
  [Host]     : .NET 11.0.0 (11.0.0-preview.5.26255.101, 11.0.26.25601), X64 RyuJIT x86-64-v3

Method	Toolchain	fileSize	userBufferSize	options	Mean	Ratio	Allocated
Read	\main\corerun.exe	1024	1024	None	145.7 us	1.00	4360 B

Read	\main\corerun.exe	1024	1024	Asynchronous	173.9 us	1.00	4576 B
Read	\managed_cached\corerun.exe	1024	1024	Asynchronous	156.9 us	0.90	4464 B
Read	\native_cached\corerun.exe	1024	1024	Asynchronous	154.6 us	0.89	4440 B

Write	\main\corerun.exe	1024	1024	None	609.2 us	1.00	4360 B

Write	\main\corerun.exe	1024	1024	Asynchronous	639.3 us	1.00	4577 B
Write	\managed_cached\corerun.exe	1024	1024	Asynchronous	627.9 us	0.98	4465 B
Write	\native_cached\corerun.exe	1024	1024	Asynchronous	623.3 us	0.98	4441 B

Read	\main\corerun.exe	1048576	512	None	779.2 us	1.00	4361 B

Read	\main\corerun.exe	1048576	512	Asynchronous	2,756.3 us	1.00	49460 B
Read	\managed_cached\corerun.exe	1048576	512	Asynchronous	1,262.3 us	0.46	4466 B
Read	\native_cached\corerun.exe	1048576	512	Asynchronous	1,113.6 us	0.40	4442 B

Write	\main\corerun.exe	1048576	512	None	7,438.8 us	1.00	4365 B

Write	\main\corerun.exe	1048576	512	Asynchronous	7,788.8 us	1.00	49470 B
Write	\managed_cached\corerun.exe	1048576	512	Asynchronous	7,507.1 us	0.96	4473 B
Write	\native_cached\corerun.exe	1048576	512	Asynchronous	7,387.7 us	0.95	4449 B

Read	\main\corerun.exe	1048576	4096	None	709.8 us	1.00	241 B

Read	\main\corerun.exe	1048576	4096	Asynchronous	2,929.2 us	1.00	45340 B
Read	\managed_cached\corerun.exe	1048576	4096	Asynchronous	1,235.6 us	0.42	346 B
Read	\native_cached\corerun.exe	1048576	4096	Asynchronous	1,049.8 us	0.36	322 B

Write	\main\corerun.exe	1048576	4096	None	7,197.4 us	1.00	245 B

Write	\main\corerun.exe	1048576	4096	Asynchronous	7,838.3 us	1.00	45350 B
Write	\managed_cached\corerun.exe	1048576	4096	Asynchronous	7,351.9 us	0.94	353 B
Write	\native_cached\corerun.exe	1048576	4096	Asynchronous	7,477.0 us	0.96	329 B

Read	\main\corerun.exe	104857600	4096	None	82,969.7 us	1.00	352 B

Read	\main\corerun.exe	104857600	4096	Asynchronous	329,198.7 us	1.00	4506328 B
Read	\managed_cached\corerun.exe	104857600	4096	Asynchronous	154,884.8 us	0.47	568 B
Read	\native_cached\corerun.exe	104857600	4096	Asynchronous	118,937.8 us	0.36	544 B

Write	\main\corerun.exe	104857600	4096	None	172,746.9 us	1.00	464 B

Write	\main\corerun.exe	104857600	4096	Asynchronous	458,918.6 us	1.00	4506328 B
Write	\managed_cached\corerun.exe	104857600	4096	Asynchronous	208,779.5 us	0.46	792 B
Write	\native_cached\corerun.exe	104857600	4096	Asynchronous	170,257.2 us	0.37	768 B

adamsitnik

@copilot please address the feedback

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot · 2026-06-09T12:47:45Z

@copilot please address the feedback

Implemented in 569ccce. The remaining feedback items are addressed: stack NativeOverlapped plus DangerousAddRef/DangerousRelease guarding in sync-over-async paths.

This reverts commit 569ccce.

…iveOverlapped.EventHandle

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

Copilot

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Initial plan

4d6f586

Copilot AI assigned Copilot and adamsitnik Apr 13, 2026

Copilot AI requested review from Copilot and removed request for Copilot April 13, 2026 17:28

Copilot started work on behalf of adamsitnik April 13, 2026 17:28 View session

Copilot AI linked an issue Apr 13, 2026 that may be closed by this pull request

Don't use callbacks when performing sync IO with async handles #126839

Open

github-actions Bot added the area-VM-coreclr label Apr 13, 2026

Copilot AI requested review from Copilot and removed request for Copilot April 13, 2026 17:58

Copilot AI changed the title ~~[WIP] Remove callbacks for sync IO operations with async handles~~ Don't use callbacks when performing sync IO with async handles Apr 13, 2026

Copilot finished work on behalf of adamsitnik April 13, 2026 18:04

Copilot AI requested a review from adamsitnik April 13, 2026 18:04

jkotas added area-System.IO and removed area-VM-coreclr labels Apr 13, 2026

build-analysis Bot mentioned this pull request Apr 14, 2026

System.Net.NameResolution.Tests DNS failures: Name or service not known #126641

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adamsitnik added this to the 11.0.0 milestone Apr 14, 2026

adamsitnik reviewed Apr 14, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

Copilot started work on behalf of adamsitnik April 14, 2026 05:24 View session

Address review nits: use target-typed new and rename AllocNativeOverl…

a34ad37

…appedWithEventHandle Agent-Logs-Url: https://github.com/dotnet/runtime/sessions/bcdd8838-718e-4561-abf4-90c9c2deb905 Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot April 14, 2026 05:47

Copilot finished work on behalf of adamsitnik April 14, 2026 05:48

Copilot AI requested a review from adamsitnik April 14, 2026 05:48

adamsitnik marked this pull request as ready for review April 14, 2026 05:53

Copilot AI review requested due to automatic review settings April 14, 2026 05:53

Copilot started reviewing on behalf of adamsitnik April 14, 2026 05:54 View session

Merge branch 'main' into copilot/remove-callbacks-sync-io

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Copilot AI reviewed Jun 8, 2026

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adamsitnik approved these changes Jun 8, 2026

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adamsitnik requested a review from jkotas June 8, 2026 16:25

build-analysis Bot mentioned this pull request Jun 8, 2026

CI build failure: error CS8032: An instance of analyzer Microsoft.NetCore.CSharp.Analyzers.... cannot be created #129031

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jkotas reviewed Jun 8, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

jkotas reviewed Jun 8, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

adamsitnik reviewed Jun 9, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

Copilot started work on behalf of adamsitnik June 9, 2026 12:37 View session

Use stack NativeOverlapped and add SafeHandle AddRef guards

569ccce

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot finished work on behalf of adamsitnik June 9, 2026 12:53

Copilot AI requested a review from adamsitnik June 9, 2026 12:54

jkotas reviewed Jun 9, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs Outdated

adamsitnik added 2 commits June 9, 2026 17:23

Revert "Use stack NativeOverlapped and add SafeHandle AddRef guards"

9d8f0f8

This reverts commit 569ccce.

use DangerousAddRef+DangerousRelease for SafeWaitHandle passed to Nat…

cfac97c

…iveOverlapped.EventHandle

Copilot AI review requested due to automatic review settings June 9, 2026 15:32

Copilot AI reviewed Jun 9, 2026

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address feedback: keep SFH alive for whole overlapped operation

b713596

build-analysis Bot mentioned this pull request Jun 9, 2026

"We stopped hearing from agent Azure Pipelines 32. Verify the agent machine is running and has a healthy network connection" dotnet/dnceng#1886

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Copilot AI reviewed Jun 9, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs

Comment thread src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/RandomAccess/Mixed.Windows.cs

This was referenced Jun 9, 2026

slow macOS - "##[error]The job running on agent Azure Pipelines 9 ran longer than the maximum time of 60 minutes." dotnet/dnceng#1883

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The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

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address code reviewer feedback

8d5ec30

Copilot AI reviewed Jun 10, 2026

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jkotas approved these changes Jun 10, 2026

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Merge branch 'main' into copilot/remove-callbacks-sync-io

cb8638f

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