Fix concurrent Unix named mutex creation by jkoritzinsky · Pull Request #134111 · dotnet/runtime

jkoritzinsky · 2026-09-16T22:56:52Z

Why

Concurrent shared compilations on Linux can intermittently fail when a newly created named mutex is initially owned. The creator can lose the mutex between publishing the shared state and recording ownership, causing a later ReleaseMutex to fail with "Cannot release a lock that is not owned by the current thread."

What changed

Keep the creation/deletion process lock held through nonblocking initial mutex acquisition and ownership bookkeeping.
Preserve the existing behavior of releasing that lock before blocking waits.
Add a Unix regression test covering concurrent same-name constructors with initial ownership.

Testing

./build.sh clr+libs -rc checked
./build.sh clr.corelib+clr.nativecorelib+libs.pretest -rc checked
dotnet test src/libraries/System.Threading/tests/System.Threading.Tests.csproj --no-build (714 passed)
Focused regression test passed.

Resolves #134043

Note

This pull request description was generated with GitHub Copilot assistance.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

azure-pipelines · 2026-09-16T22:57:32Z

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dotnet-policy-service · 2026-09-16T22:57:58Z

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🟡 Changes recommended

Add platform capability guards to prevent unsupported targets from failing or hanging.

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Findings: 1

Open (1)

Skip platforms without Unix named-mutex multithreading support · New

What changed in this PR

Fixes a Unix named-mutex creation race by preserving the process lock through initial acquisition and ownership bookkeeping, with a concurrent regression test.

Changes:

Updates Unix mutex acquisition locking.
Adds concurrent initial-ownership coverage.
Test requires platform capability guards for unsupported targets.

File	Description
`src/libraries/System.Threading/tests/MutexTests.cs`	Adds concurrent named-mutex constructor coverage; requires guards for mobile, Browser, and Wasi.
`src/libraries/System.Private.CoreLib/src/System/Threading/NamedMutex.Unix.cs`	Preserves locking during nonblocking ownership setup.

Co-authored-by: jkoritzinsky <1571408+jkoritzinsky@users.noreply.github.com>

Copilot

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🟢 Approval recommended

No unresolved blocking issues were identified.

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Resolved since last review (1)

Skip platforms without Unix named-mutex multithreading support

jkoritzinsky · 2026-09-17T17:44:35Z

/ba-g JSON oom unrelated

…es the mutex. Preserve the assert as an exception-throw in release builds instead of a debug-only assert.

Copilot

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🟡 Changes recommended

Add actionable context to the new invariant exception before approval.

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Open (1)

Include context when the new-mutex acquisition invariant fails · New

Copilot

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🟡 Changes recommended

Unresolved mutex acquisition and regression-test issues remain.

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Findings: 1

Open (1)

Preserve abandoned mutex acquisition results · New

Previously missed (1)

In code that hasn't changed since last review

Avoid process lock for timed-out or recursive waits

src/libraries/System.Private.CoreLib/src/System/Threading/NamedMutex.Unix.cs:672

This acquires the global creation/deletion process lock for every result, even though RecordMutexAcquisition immediately returns for TimedOut and AcquiredLockRecursively. A timed-out or recursive wait can therefore block behind unrelated named-mutex creation/deletion work despite doing no bookkeeping; only acquire the process lock when the raw result needs ownership recording.

Copilot

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🔵 Needs a closer look

A moderate issue affects the nonblocking behavior of zero-timeout waits, and a minor unused local remains.

Review effort: Lite
Findings: None

Resolved since last review (1)

Preserve abandoned mutex acquisition results

agocke · 2026-09-23T22:12:35Z

Can you file a bug for the mono issue? In case we need to know what the problem is in the future?

jkoritzinsky · 2026-09-23T22:18:38Z

I think I may have addressed the Mono issue with the other fix. I'll revert the Mono disable and have CI check it out for me.

…nized"

Copilot

Copilot review overview

🔵 Needs a closer look

Timed and nonblocking acquisition must avoid blocking on the creation lock.

Review effort: Lite
Findings: None

jkoritzinsky · 2026-09-23T23:22:57Z

Yep the runs on Mono caught the bug because the Thread.Join implementation there is less robust.

We're good to go.

jkoritzinsky · 2026-09-24T16:41:03Z

/ba-g ssl stream tests unrelated

jkoritzinsky · 2026-09-24T16:41:13Z

/backport to release/11.0

github-actions · 2026-09-24T16:41:23Z

Started backporting to release/11.0 (link to workflow run)

@jkoritzinsky

Backport of #134111 to release/11.0 /cc @jkoritzinsky ## Customer Impact - [x] Customer reported - [ ] Found internally Creating a new named mutex with the `initiallyOwned` parameter set to `true` from two threads in the same process could allow one of the threads to proceed without owning the mutex on Unix platforms. This was found by Roslyn in their move to use multithreaded MSBuild tasks. ## Regression - [X] Yes - [ ] No #117635 ## Testing Unit test added that forces this particular race many times in a loop until it occurs. ## Risk Low, only touches named mutex logic and the test validates the exact failure case reliably (always hits in the first 150 iterations on failure). Co-authored-by: Jeremy Koritzinsky <jekoritz@microsoft.com> Co-authored-by: Jeremy Koritzinsky <Jeremy.Koritzinsky@microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: jkoritzinsky <1571408+jkoritzinsky@users.noreply.github.com>

Fix concurrent Unix named mutex creation

4d58f14

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI lite review requested due to automatic review settings September 16, 2026 22:56

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Comment thread src/libraries/System.Threading/tests/MutexTests.cs Outdated

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This was referenced Sep 17, 2026

AF: *(_UNCHECKED_OBJECTREF *)handle == NULL (HndCreateHandle called by getJitHandleForObject) #117138

Open

[ci-scan] Test failure: System.Net.WebSockets.Tests.WebSocketKeepAliveTests.WebSocketServer_NoUserReadOrWrite_SendsPingAndReadsPongResponse #133374

Open

Guard named mutex test capabilities

260ea1c

Co-authored-by: jkoritzinsky <1571408+jkoritzinsky@users.noreply.github.com>

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Test failure: GC/Features/HeapExpansion/bestfit-finalize/bestfit-finalize.cmd #132107

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WebSocketTests.ReceiveAsync_WhenDisposedInParallel_DoesNotGetStuck fails with unexpected exception #133446

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Comment thread src/libraries/System.Threading/tests/MutexTests.cs Outdated

agocke reviewed Sep 17, 2026

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Comment thread src/libraries/System.Private.CoreLib/src/System/Threading/NamedMutex.Unix.cs Outdated

Adjust test to actually ensure that the creating thread always captur…

3671d1c

…es the mutex. Preserve the assert as an exception-throw in release builds instead of a debug-only assert.

jkoritzinsky requested review from agocke and Copilot September 17, 2026 21:57

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Comment thread src/libraries/System.Private.CoreLib/src/System/Threading/NamedMutex.Unix.cs

build-analysis Bot mentioned this pull request Sep 18, 2026

XHarness package install failure on iOS due to devicectl NSPOSIXErrorDomain error 49 #123796

Open

jkoritzinsky requested a review from AaronRobinsonMSFT September 22, 2026 18:48

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Comment thread src/libraries/System.Private.CoreLib/src/System/Threading/NamedMutex.Unix.cs Outdated

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Comment thread src/libraries/System.Private.CoreLib/src/System/Threading/NamedMutex.Unix.cs Outdated

Copilot AI review requested due to automatic review settings September 23, 2026 21:50

Copilot started reviewing on behalf of jkoritzinsky September 23, 2026 21:51 View session

Return raw acquire result in NamedMutex

d8d7a8b

jkoritzinsky force-pushed the dev/jkoritzinsky/shiny-adventure branch from 29f591c to d8d7a8b Compare September 23, 2026 21:52

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Don't skip on mono now that we fixed "native abandonment is not recog…

4a6cef1

…nized"

Copilot AI review requested due to automatic review settings September 23, 2026 22:22

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jkoritzinsky requested a review from AaronRobinsonMSFT September 24, 2026 02:18

build-analysis Bot mentioned this pull request Sep 24, 2026

System.Security.Cryptography.Tests.HpkeImplementationTests.InvalidEncapsulation_Rejected fails in CI #134528

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jkoritzinsky disabled auto-merge September 24, 2026 16:41

jkoritzinsky merged commit 9a4b4fc into main Sep 24, 2026
141 of 143 checks passed

jkoritzinsky deleted the dev/jkoritzinsky/shiny-adventure branch September 24, 2026 16:41

github-project-automation Bot moved this to Done in AppModel Sep 24, 2026

github-actions Bot mentioned this pull request Sep 24, 2026

[release/11.0] Fix concurrent Unix named mutex creation #134596

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dotnet-maestro Bot mentioned this pull request Sep 25, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#9743

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jkoritzinsky commented Sep 16, 2026

Why

What changed

Testing

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