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Make SpinLock.TryEnter(ref bool) fail faster #2628

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jkotas merged 2 commits into from
Feb 1, 2017
Merged

Make SpinLock.TryEnter(ref bool) fail faster #2628

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5fc571c
Make SpinLock.TryEnter(ref bool) fail faster
stephentoub Jan 31, 2017
c161443
Port cosmetic SpinLock changes from CoreCLR
jkotas Feb 1, 2017
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47 changes: 29 additions & 18 deletions src/System.Private.CoreLib/src/System/Threading/SpinLock.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,17 +5,15 @@

// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// SpinLock.cs
// A spin lock is a mutual exclusion lock primitive where a thread trying to acquire the lock waits in a loop ("spins")
// repeatedly checking until the lock becomes available. As the thread remains active performing a non-useful task,
// the use of such a lock is a kind of busy waiting and consumes CPU resources without performing real work.
//

//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

using Internal.Threading.Tracing;

Expand Down Expand Up @@ -104,7 +102,7 @@ public struct SpinLock
// The waiters count is calculated by m_owner & WAITERS_MASK 01111....110
private static int MAXIMUM_WAITERS = WAITERS_MASK;


[MethodImpl(MethodImplOptions.AggressiveInlining)]
private int CompareExchange(ref int location, int value, int comparand, ref bool success)
{
int result = Interlocked.CompareExchange(ref location, value, comparand);
Expand All @@ -131,7 +129,6 @@ public SpinLock(bool enableThreadOwnerTracking)
}
}


/// <summary>
/// Initializes a new instance of the <see cref="T:System.Threading.SpinLock"/>
/// structure with the option to track thread IDs to improve debugging.
Expand Down Expand Up @@ -189,7 +186,22 @@ public void Enter(ref bool lockTaken)
/// </exception>
public void TryEnter(ref bool lockTaken)
{
TryEnter(0, ref lockTaken);
int observedOwner = m_owner;
if (((observedOwner & LOCK_ID_DISABLE_MASK) == 0) | lockTaken)
{
// Thread tracking enabled or invalid arg. Take slow path.
ContinueTryEnter(0, ref lockTaken);
}
else if ((observedOwner & LOCK_ANONYMOUS_OWNED) != 0)
{
// Lock already held by someone
lockTaken = false;
}
else
{
// Lock wasn't held; try to acquire it.
CompareExchange(ref m_owner, observedOwner | LOCK_ANONYMOUS_OWNED, observedOwner, ref lockTaken);
}
}

/// <summary>
Expand Down Expand Up @@ -286,7 +298,6 @@ private void ContinueTryEnter(int millisecondsTimeout, ref bool lockTaken)
nameof(millisecondsTimeout), millisecondsTimeout, SR.SpinLock_TryEnter_ArgumentOutOfRange);
}


uint startTime = 0;
if (millisecondsTimeout != Timeout.Infinite && millisecondsTimeout != 0)
{
Expand Down Expand Up @@ -323,10 +334,15 @@ private void ContinueTryEnter(int millisecondsTimeout, ref bool lockTaken)
observedOwner = m_owner;
if ((observedOwner & LOCK_ANONYMOUS_OWNED) == LOCK_UNOWNED)
{
if (CompareExchange(ref m_owner, observedOwner | 1, observedOwner, ref lockTaken) == observedOwner
|| millisecondsTimeout == 0)
if (CompareExchange(ref m_owner, observedOwner | 1, observedOwner, ref lockTaken) == observedOwner)
{
// Aquired lock
return;
}

if (millisecondsTimeout == 0)
{
// Aquired lock, or did not aquire lock as owned but timeout is 0 so fail fast
// Did not aquire lock in CompareExchange and timeout is 0 so fail fast
return;
}
}
Expand Down Expand Up @@ -496,7 +512,6 @@ private void ContinueTryEnterWithThreadTracking(int millisecondsTimeout, uint st
/// <exception cref="SynchronizationLockException">
/// Thread ownership tracking is enabled, and the current thread is not the owner of this lock.
/// </exception>
//[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
public void Exit()
{
//This is the fast path for the thread tracking is disabled, otherwise go to the slow path
Expand All @@ -522,15 +537,14 @@ public void Exit()
/// <exception cref="SynchronizationLockException">
/// Thread ownership tracking is enabled, and the current thread is not the owner of this lock.
/// </exception>
//[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
public void Exit(bool useMemoryBarrier)
{
// This is the fast path for the thread tracking is diabled and not to use memory barrier, otherwise go to the slow path
// The reason not to add else statement if the usememorybarrier is that it will add more barnching in the code and will prevent
// method inlining, so this is optimized for useMemoryBarrier=false and Exit() overload optimized for useMemoryBarrier=true
if ((m_owner & LOCK_ID_DISABLE_MASK) != 0 && !useMemoryBarrier)
// method inlining, so this is optimized for useMemoryBarrier=false and Exit() overload optimized for useMemoryBarrier=true.
int tmpOwner = m_owner;
if ((tmpOwner & LOCK_ID_DISABLE_MASK) != 0 & !useMemoryBarrier)
{
int tmpOwner = m_owner;
m_owner = tmpOwner & (~LOCK_ANONYMOUS_OWNED);
}
else
Expand Down Expand Up @@ -577,7 +591,6 @@ private void ExitSlowPath(bool useMemoryBarrier)
/// </summary>
public bool IsHeld
{
//[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
get
{
if (IsThreadOwnerTrackingEnabled)
Expand All @@ -603,7 +616,6 @@ public bool IsHeld
/// </exception>
public bool IsHeldByCurrentThread
{
//[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
get
{
if (!IsThreadOwnerTrackingEnabled)
Expand All @@ -617,7 +629,6 @@ public bool IsHeldByCurrentThread
/// <summary>Gets whether thread ownership tracking is enabled for this instance.</summary>
public bool IsThreadOwnerTrackingEnabled
{
//[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
get { return (m_owner & LOCK_ID_DISABLE_MASK) == 0; }
}

Expand Down