Revert "A few fixes in the threadpool semaphore. Unify Windows/Unix implementation of LIFO policy."

[agocke]

Reverts #123921

This change appears to have caused a large regression in NuGet restore performance. Reverting is confirmed to produce a significant improvement (10-15%).

[dotnet-policy-service]

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[Copilot]

Pull request overview

This PR reverts PR #123921 which had unified the Windows/Unix LIFO semaphore implementation and introduced futex-based thread blocking. The revert is motivated by a confirmed 10-15% regression in NuGet restore performance caused by the original change. The revert restores the original platform-specific LowLevelLifoSemaphore implementations (IO Completion Port on Windows, WaitSubsystem on Unix), removes the futex infrastructure (LowLevelFutex, LowLevelThreadBlocker), and reverts the spinning heuristics and Backoff.Exponential changes.

Changes:

• Restores platform-specific LowLevelLifoSemaphore implementations (LowLevelLifoSemaphore.Windows.cs using IO Completion Ports, LowLevelLifoSemaphore.Unix.cs using WaitSubsystem) and removes the unified futex-based thread blocking infrastructure.
• Reverts the thread pool semaphore spinning strategy from an adaptive per-core-availability approach back to a simple configurable spin count with random exponential backoff.
• Removes the Synchronization.lib dependency and WaitOnAddress/WakeByAddressSingle Win32 API usage across managed and native code.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_threading.h	Removes futex function declarations
src/native/libs/System.Native/pal_threading.c	Removes futex function implementations (Linux syscall + fallback)
src/native/libs/System.Native/entrypoints.c	Removes futex DllImport entries
src/libraries/.../PortableThreadPool.WorkerThread.cs	Reverts semaphore Wait call to 1-param, restores spin count config at call site
src/libraries/.../LowLevelThreadBlocker.cs	Deletes unified thread blocker class
src/libraries/.../LowLevelLifoSemaphore.cs	Reverts to platform-dispatching partial class with simpler spin logic
src/libraries/.../LowLevelLifoSemaphore.Windows.cs	Restores IOCP-based Windows implementation
src/libraries/.../LowLevelLifoSemaphore.Unix.cs	Restores WaitSubsystem-based Unix implementation
src/libraries/.../LowLevelFutex.Windows.cs	Deletes Windows futex wrapper
src/libraries/.../LowLevelFutex.Unix.cs	Deletes Unix futex wrapper
src/libraries/.../Backoff.cs	Reverts to void return, removes attempt>0 guard, restores max bits to 14
src/libraries/.../System.Private.CoreLib.Shared.projitems	Updates build item includes for restored/removed files
src/libraries/.../Interop.WaitOnAddress.cs	Deletes WaitOnAddress/WakeByAddressSingle P/Invoke
src/libraries/.../Interop.CriticalSection.cs	Removes [SuppressGCTransition] from LeaveCriticalSection
src/libraries/.../Interop.Libraries.cs	Removes Synch library constant, adds duplicate Oleaut32
src/libraries/.../Interop.LowLevelMonitor.cs	Removes [SuppressGCTransition] from LowLevelMonitor_Release
src/libraries/.../Interop.Futex.cs	Deletes futex P/Invoke declarations
src/coreclr/.../reproNative.vcxproj	Removes Synchronization.lib from SDK libs
src/coreclr/.../WindowsAPIs.txt	Removes WaitOnAddress/WakeByAddressSingle API entries
src/coreclr/.../Microsoft.NETCore.Native.Windows.targets	Removes Synchronization.lib
docs/coding-guidelines/interop-guidelines.md	Reverts documentation examples to pre-#123921 state

[agocke]

/ba-g known issue filed