From 0855b70b2542de60592cba7fe1c91bedb55566f4 Mon Sep 17 00:00:00 2001
From: Ben Adams <thundercat@illyriad.co.uk>
Date: Fri, 14 Oct 2016 15:19:41 +0100
Subject: [PATCH] SpinLock.TryEnter fail fast for timeout 0 (#6952)

* SpinLock.TryEnter fail fast for timeout 0 (2)

* Don't over check SpinLock timeout

When 1ms unlikely to have passed
---
 src/mscorlib/src/System/Threading/SpinLock.cs | 35 ++++++++++++++-------------
 1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/src/mscorlib/src/System/Threading/SpinLock.cs b/src/mscorlib/src/System/Threading/SpinLock.cs
index dea8743..9f5cf68 100644
--- a/src/mscorlib/src/System/Threading/SpinLock.cs
+++ b/src/mscorlib/src/System/Threading/SpinLock.cs
@@ -339,12 +339,23 @@ namespace System.Threading
 
                 if (Interlocked.CompareExchange(ref m_owner, observedOwner | 1, observedOwner, ref lockTaken) == observedOwner)
                 {
+                    // Aquired lock
                     return;
                 }
 
 #if !FEATURE_CORECLR
                 Thread.EndCriticalRegion();
 #endif
+                if (millisecondsTimeout == 0)
+                {
+                    // Did not aquire lock in CompareExchange and timeout is 0 so fail fast
+                    return;
+                }
+            }
+            else if (millisecondsTimeout == 0)
+            {
+                // Did not aquire lock as owned and timeout is 0 so fail fast
+                return;
             }
             else //failed to acquire the lock,then try to update the waiters. If the waiters count reached the maximum, jsut break the loop to avoid overflow
             {
@@ -352,17 +363,6 @@ namespace System.Threading
                     turn = (Interlocked.Add(ref m_owner, 2) & WAITERS_MASK) >> 1 ;
             }
 
-
-
-            // Check the timeout.
-            if (millisecondsTimeout == 0 ||
-                (millisecondsTimeout != Timeout.Infinite &&
-                TimeoutHelper.UpdateTimeOut(startTime, millisecondsTimeout) <= 0))
-            {
-                DecrementWaiters();
-                return;
-            }
-
             //***Step 2. Spinning
             //lock acquired failed and waiters updated
             int processorCount = PlatformHelper.ProcessorCount;
@@ -396,15 +396,16 @@ namespace System.Threading
 #endif
                     }
                 }
-            }
 
-            // Check the timeout.
-            if (millisecondsTimeout != Timeout.Infinite && TimeoutHelper.UpdateTimeOut(startTime, millisecondsTimeout) <= 0)
-            {
-                DecrementWaiters();
-                return;
+                // Check the timeout.
+                if (millisecondsTimeout != Timeout.Infinite && TimeoutHelper.UpdateTimeOut(startTime, millisecondsTimeout) <= 0)
+                {
+                    DecrementWaiters();
+                    return;
+                }
             }
 
+
             //*** Step 3, Yielding
             //Sleep(1) every 50 yields
             int yieldsoFar = 0;
-- 
2.7.4