Remove CoreCLR PAL semaphore support

[jkoritzinsky]

CoreCLR no longer needs PAL semaphore objects: Unix System.Threading.Semaphore uses the managed wait subsystem, and metadata's legacy UTSemReadWrite was the remaining PAL consumer. Removing this support reduces the PAL synchronization surface and its associated object-management code.

This change:

• adds a minipal read-write lock backed by SRWLOCK on Windows and pthread_rwlock_t elsewhere, requesting writer preference where supported;
• replaces metadata's UTSemReadWrite usage while preserving lock contracts, debugger bookkeeping, ownership transfers, and debug assertions; and
• removes the PAL semaphore API, object implementation, exports, compatibility constants, and dedicated tests.

The POSIX semaphores used by debugger startup coordination in process.cpp remain; they are independent of PAL semaphore objects.

Testing

• ./build.sh clr+libs+host
• ./build.sh clr.runtime+clr.paltests
• ./build.sh clr.runtime -rc Release
• PAL test suite: 267 passed, 0 failed
• multithreaded minipal read-write lock contention stress test
• Reflection.Emit metadata-lock smoke test under the rebuilt Debug runtime

Performance

BenchmarkDotNet comparison using baseline and changed Release CoreRun hosts, with 2 launches, 5 warmup iterations, and 15 measurement iterations of 250 ms:

Scenario	Baseline	Changed	Difference
Read metadata	1.974 us	1.965 us	-0.5%
Write metadata	267.846 us	259.448 us	-3.1%

Benchmark source

using System.Reflection;
using System.Reflection.Emit;
using BenchmarkDotNet.Attributes;

[MemoryDiagnoser]
public class MetadataLockBench
{
    private Assembly _assembly = null!;
    private int _typeId;

    [GlobalSetup]
    public void Setup()
    {
        AssemblyBuilder assembly = AssemblyBuilder.DefineDynamicAssembly(
            new AssemblyName("MetadataReadBenchmark"),
            AssemblyBuilderAccess.Run);
        ModuleBuilder module = assembly.DefineDynamicModule("MetadataReadBenchmark");

        for (int i = 0; i < 100; i++)
        {
            TypeBuilder type = module.DefineType($"Type{i}", TypeAttributes.Public);
            type.DefineField("Value", typeof(int), FieldAttributes.Public);
            type.CreateType();
        }

        _assembly = assembly;
    }

    [Benchmark]
    public Type[] ReadMetadata()
    {
        return _assembly.GetTypes();
    }

    [Benchmark]
    public Type WriteMetadata()
    {
        int typeId = Interlocked.Increment(ref _typeId);
        AssemblyBuilder assembly = AssemblyBuilder.DefineDynamicAssembly(
            new AssemblyName($"MetadataWriteBenchmark{typeId}"),
            AssemblyBuilderAccess.RunAndCollect);
        ModuleBuilder module = assembly.DefineDynamicModule("MetadataWriteBenchmark");
        TypeBuilder type = module.DefineType("EmittedType", TypeAttributes.Public);

        for (int i = 0; i < 10; i++)
        {
            type.DefineField($"Field{i}", typeof(int), FieldAttributes.Public);
        }

        return type.CreateType()!;
    }
}

Note

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[Copilot]

Pull request overview

This PR removes CoreCLR PAL “semaphore object” support (implementation, exports, constants, and PAL tests) and replaces metadata’s legacy UTSemReadWrite usage with a new minipal_rwlock abstraction (SRWLOCK on Windows, pthread_rwlock_t elsewhere, preferring writers when supported). The intent is to shrink the PAL synchronization surface while preserving metadata locking contracts and debug bookkeeping.

Changes:

• Add minipal_rwlock (new minipal rwlock header + implementation, plus configure-time feature detection and build wiring).
• Migrate metadata locking from UTSemReadWrite to minipal_rwlock, including updated helper APIs/macros and debug write-lock tracking.
• Remove PAL semaphore object support and associated PAL semaphore tests, lists, exports, and error constants.

Reviewed changes

Copilot reviewed 45 out of 45 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/native/minipal/rwlock.h	Adds the minipal_rwlock public header (SRWLOCK/pthread-backed).
src/native/minipal/rwlock.c	Implements rwlock init/enter/leave for Windows and pthread platforms.
src/native/minipal/minipalconfig.h.in	Adds HAVE_PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP config define.
src/native/minipal/configure.cmake	Adds compile-time probe for writer-preference rwlock attribute support.
src/native/minipal/CMakeLists.txt	Includes rwlock.c in minipal sources.
src/coreclr/vm/gccover.cpp	Removes now-unused utsem.h include.
src/coreclr/utilcode/utsem.cpp	Deletes the legacy UTSemReadWrite implementation.
src/coreclr/utilcode/CMakeLists.txt	Removes utsem.cpp from utilcode build sources.
src/coreclr/pal/tests/palsuite/threading/WaitForSingleObject/WFSOSemaphoreTest/WFSOSemaphoreTest.cpp	Removes PAL semaphore WFSO test (PAL semaphore objects removed).
src/coreclr/pal/tests/palsuite/threading/releasesemaphore/test1/test.cpp	Removes PAL ReleaseSemaphore test (PAL semaphore objects removed).
src/coreclr/pal/tests/palsuite/threading/DuplicateHandle/test10/test10.cpp	Removes PAL DuplicateHandle(semaphore) test (PAL semaphore objects removed).
src/coreclr/pal/tests/palsuite/threading/CreateSemaphoreW_ReleaseSemaphore/test3/createsemaphore.cpp	Removes PAL CreateSemaphore/ReleaseSemaphore test (PAL semaphore objects removed).
src/coreclr/pal/tests/palsuite/threading/CreateSemaphoreW_ReleaseSemaphore/test2/CreateSemaphore.cpp	Removes PAL CreateSemaphore/ReleaseSemaphore test (PAL semaphore objects removed).
src/coreclr/pal/tests/palsuite/threading/CreateSemaphoreW_ReleaseSemaphore/test1/CreateSemaphore.cpp	Removes PAL CreateSemaphore/ReleaseSemaphore test (PAL semaphore objects removed).
src/coreclr/pal/tests/palsuite/paltestlist.txt	Removes semaphore-related entries from PAL test list.
src/coreclr/pal/tests/palsuite/paltestlist_to_be_reviewed.txt	Removes semaphore-related entry from “to be reviewed” list.
src/coreclr/pal/tests/palsuite/compilableTests.txt	Removes semaphore-related entries from compilable tests list.
src/coreclr/pal/tests/palsuite/CMakeLists.txt	Removes semaphore-related PAL test sources from build.
src/coreclr/pal/src/synchobj/semaphore.cpp	Deletes PAL semaphore object implementation.
src/coreclr/pal/src/synchmgr/wait.cpp	Removes semaphore from allowed wait/signalable PAL object types.
src/coreclr/pal/src/misc/errorstrings.cpp	Removes semaphore-timeout error string mapping.
src/coreclr/pal/src/include/pal/semaphore.hpp	Deletes PAL semaphore internal header.
src/coreclr/pal/src/include/pal/corunix.hpp	Removes otiSemaphore PAL object type id.
src/coreclr/pal/src/CMakeLists.txt	Removes semaphore object source from PAL build.
src/coreclr/pal/inc/pal.h	Removes PAL semaphore APIs and semaphore access constants from public PAL header.
src/coreclr/pal/inc/pal_error.h	Removes PAL-only semaphore-related error codes.
src/coreclr/md/runtime/mdinternalro.h	Updates reader-writer lock type to minipal_rwlock* in MDInternalRO contract.
src/coreclr/md/inc/rwutil.h	Introduces minipal_rwlock-based lock helpers and updates lock holder/macros.
src/coreclr/md/inc/metamodelrw.h	Replaces debug lock pointer tracking with explicit “write-locked” flag tracking.
src/coreclr/md/inc/mdinternalrw.h	Updates MDInternalRW reader-writer lock APIs to use minipal_rwlock*.
src/coreclr/md/enc/stdafx.h	Removes utsem.h include from metadata ENC build.
src/coreclr/md/enc/rwutil.cpp	Implements metadata lock helpers (CreateMDReadWriteLock, AcquireMD*Lock, etc.) and updates lock holder class.
src/coreclr/md/enc/metamodelrw.cpp	Updates debug write-lock assertion to use the new flag-based tracking.
src/coreclr/md/enc/mdinternalrw.cpp	Updates call sites to acquire/release metadata write lock using new helpers/types.
src/coreclr/md/compiler/stdafx.h	Removes utsem.h include from metadata compiler build.
src/coreclr/md/compiler/regmeta.h	Updates RegMeta lock type and lock wiring to minipal_rwlock*.
src/coreclr/md/compiler/regmeta.cpp	Updates RegMeta lock allocation/teardown and lock usage to minipal_rwlock helpers.
src/coreclr/md/compiler/mdutil.h	Updates global loaded-modules lock type to minipal_rwlock*.
src/coreclr/md/compiler/mdutil.cpp	Updates global loaded-modules lock initialization and lock usage to minipal_rwlock.
src/coreclr/inc/winwrap.h	Removes PAL-specific CreateSemaphore wrapper macro.
src/coreclr/inc/utsem.h	Deletes legacy UTSemReadWrite header.
src/coreclr/inc/metadata.h	Updates internal metadata interfaces to use minipal_rwlock* instead of UTSemReadWrite*.
src/coreclr/inc/corpriv.h	Updates IMetaDataHelper lock APIs to use minipal_rwlock*.
src/coreclr/inc/corhost.h	Removes local CreateSemaphore undef logic tied to PAL semaphore mapping.
src/coreclr/dlls/mscordac/mscordac_unixexports.src	Removes PAL semaphore exports from DAC unix export list.

[Copilot]

Pull request overview

Copilot reviewed 45 out of 45 changed files in this pull request and generated no new comments.

Suppressed comments (2)

Previously missed (1) — in code that hasn't changed since the last review.

src/coreclr/md/enc/rwutil.cpp:1294

• The comment block above CMDReadWriteLock::LockRead() says it obtains the "write lock", but this method acquires the read/shared lock. This is misleading when debugging lock-mode issues in metadata code.

This issue also appears on line 1318 of the same file.

// Used to obtain the write lock

src/coreclr/md/enc/rwutil.cpp:1318

• The comment block above CMDReadWriteLock::LockWrite() says it obtains the "read lock", but this method acquires the write/exclusive lock. Keeping these comment headers accurate helps avoid confusion when tracing lock ownership and contract violations.

// Used to obtain the read lock

[Copilot]

Pull request overview

Copilot reviewed 46 out of 46 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

src/native/minipal/configure.cmake:34

• The check for pthread_rwlockattr_setkind_np uses check_c_source_compiles(), which performs a link step. Because CMAKE_REQUIRED_LIBRARIES isn’t set to include the thread library, this probe can fail to link on platforms that require explicit pthread linkage (e.g., glibc), leaving HAVE_PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP unset even when the API is available.

[Copilot]

Pull request overview

Copilot reviewed 46 out of 46 changed files in this pull request and generated 1 comment.