Fix unsafe Random use in ReaderWriterLock samples (#4099)

The ReaderWriterLock samples were sharing a single Random instance
across threads. The various .Next methods on Random instances are not
safe to be called concurrently.

Now, each thread has its own Random instance that it uses. Some of the
worker methods needed to be adjusted to take a Random instance as input.

I considered using a thread-local Random instance. However, the sample
is already complex, so I opted to avoid introducing thread-local values,
which would need to be briefly explained.

Author: chwarr

samples/snippets/cpp/VS_Snippets_CLR_System/system.Threading.ReaderWriterLock/CPP/source.cpp

@@ -1,4 +1,3 @@
-
 //<Snippet1>
 // This example shows a ReaderWriterLock protecting a shared
 // resource that is read concurrently and written exclusively
@@ -23,7 +22,6 @@ public ref class Test
    //</Snippet2>
    literal int numThreads = 26;
    static bool running = true;
-   static Random^ rnd = gcnew Random;
 
    // Statistics.
    static int readerTimeouts = 0;
@@ -32,10 +30,11 @@ public ref class Test
    static int writes = 0;
    static void ThreadProc()
    {
-      
+      Random^ rnd = gcnew Random;
+
       // As long as a thread runs, it randomly selects
-      // various ways to read and write from the shared 
-      // resource. Each of the methods demonstrates one 
+      // various ways to read and write from the shared
+      // resource. Each of the methods demonstrates one
       // or more features of ReaderWriterLock.
       while ( running )
       {
@@ -44,12 +43,12 @@ public ref class Test
                   ReadFromResource( 10 );
          else
          if ( action < .81 )
-                  ReleaseRestore( 50 );
+                  ReleaseRestore( rnd, 50 );
          else
          if ( action < .90 )
-                  UpgradeDowngrade( 100 );
+                  UpgradeDowngrade( rnd, 100 );
          else
-                  WriteToResource( 100 );
+                  WriteToResource( rnd, 100 );
       }
    }
 
@@ -64,23 +63,23 @@ public ref class Test
          rwl->AcquireReaderLock( timeOut );
          try
          {
-            
+
             // It is safe for this thread to read from
             // the shared resource.
             Display( String::Format( "reads resource value {0}", resource ) );
             Interlocked::Increment( reads );
          }
          finally
          {
-            
+
             // Ensure that the lock is released.
             rwl->ReleaseReaderLock();
          }
 
       }
-      catch ( ApplicationException^ ) 
+      catch ( ApplicationException^ )
       {
-         
+
          // The reader lock request timed out.
          Interlocked::Increment( readerTimeouts );
       }
@@ -92,14 +91,14 @@ public ref class Test
    //<Snippet4>
    // Shows how to request and release the writer lock, and
    // how to handle time-outs.
-   static void WriteToResource( int timeOut )
+   static void WriteToResource( Random^ rnd, int timeOut )
    {
       try
       {
          rwl->AcquireWriterLock( timeOut );
          try
          {
-            
+
             // It is safe for this thread to read or write
             // from the shared resource.
             resource = rnd->Next( 500 );
@@ -108,15 +107,15 @@ public ref class Test
          }
          finally
          {
-            
+
             // Ensure that the lock is released.
             rwl->ReleaseWriterLock();
          }
 
       }
-      catch ( ApplicationException^ ) 
+      catch ( ApplicationException^ )
       {
-         
+
          // The writer lock request timed out.
          Interlocked::Increment( writerTimeouts );
       }
@@ -129,32 +128,32 @@ public ref class Test
    // Shows how to request a reader lock, upgrade the
    // reader lock to the writer lock, and downgrade to a
    // reader lock again.
-   static void UpgradeDowngrade( int timeOut )
+   static void UpgradeDowngrade( Random^ rnd, int timeOut )
    {
       try
       {
          rwl->AcquireReaderLock( timeOut );
          try
          {
-            
+
             // It is safe for this thread to read from
             // the shared resource.
             Display( String::Format( "reads resource value {0}", resource ) );
             Interlocked::Increment( reads );
-            
+
             // If it is necessary to write to the resource,
-            // you must either release the reader lock and 
+            // you must either release the reader lock and
             // then request the writer lock, or upgrade the
             // reader lock. Note that upgrading the reader lock
             // puts the thread in the write queue, behind any
-            // other threads that might be waiting for the 
+            // other threads that might be waiting for the
             // writer lock.
             try
             {
                LockCookie lc = rwl->UpgradeToWriterLock( timeOut );
                try
                {
-                  
+
                   // It is safe for this thread to read or write
                   // from the shared resource.
                   resource = rnd->Next( 500 );
@@ -163,36 +162,36 @@ public ref class Test
                }
                finally
                {
-                  
+
                   // Ensure that the lock is released.
                   rwl->DowngradeFromWriterLock( lc );
                }
 
             }
-            catch ( ApplicationException^ ) 
+            catch ( ApplicationException^ )
             {
-               
+
                // The upgrade request timed out.
                Interlocked::Increment( writerTimeouts );
             }
 
-            
-            // When the lock has been downgraded, it is 
+
+            // When the lock has been downgraded, it is
             // still safe to read from the resource.
             Display( String::Format( "reads resource value {0}", resource ) );
             Interlocked::Increment( reads );
          }
          finally
          {
-            
+
             // Ensure that the lock is released.
             rwl->ReleaseReaderLock();
          }
 
       }
-      catch ( ApplicationException^ ) 
+      catch ( ApplicationException^ )
       {
-         
+
          // The reader lock request timed out.
          Interlocked::Increment( readerTimeouts );
       }
@@ -207,37 +206,37 @@ public ref class Test
    // to determine whether another thread has obtained
    // a writer lock since this thread last accessed the
    // resource.
-   static void ReleaseRestore( int timeOut )
+   static void ReleaseRestore( Random^ rnd, int timeOut )
    {
       int lastWriter;
       try
       {
          rwl->AcquireReaderLock( timeOut );
          try
          {
-            
+
             // It is safe for this thread to read from
             // the shared resource. Cache the value. (You
             // might do this if reading the resource is
             // an expensive operation.)
             int resourceValue = resource;
             Display( String::Format( "reads resource value {0}", resourceValue ) );
             Interlocked::Increment( reads );
-            
+
             // Save the current writer sequence number.
             lastWriter = rwl->WriterSeqNum;
-            
+
             // Release the lock, and save a cookie so the
             // lock can be restored later.
             LockCookie lc = rwl->ReleaseLock();
-            
-            // Wait for a random interval (up to a 
+
+            // Wait for a random interval (up to a
             // quarter of a second), and then restore
             // the previous state of the lock. Note that
             // there is no timeout on the Restore method.
             Thread::Sleep( rnd->Next( 250 ) );
             rwl->RestoreLock( lc );
-            
+
             // Check whether other threads obtained the
             // writer lock in the interval. If not, then
             // the cached value of the resource is still
@@ -255,15 +254,15 @@ public ref class Test
          }
          finally
          {
-            
+
             // Ensure that the lock is released.
             rwl->ReleaseReaderLock();
          }
 
       }
-      catch ( ApplicationException^ ) 
+      catch ( ApplicationException^ )
       {
-         
+
          // The reader lock request timed out.
          Interlocked::Increment( readerTimeouts );
       }
@@ -273,7 +272,7 @@ public ref class Test
 
    //</Snippet6>
    // Helper method briefly displays the most recent
-   // thread action. Comment out calls to Display to 
+   // thread action. Comment out calls to Display to
    // get a better idea of throughput.
    static void Display( String^ msg )
    {
@@ -288,7 +287,7 @@ public ref class Test
 int main()
 {
    array<String^>^args = Environment::GetCommandLineArgs();
-   
+
    // Start a series of threads. Each thread randomly
    // performs reads and writes on the shared resource.
    array<Thread^>^t = gcnew array<Thread^>(Test::numThreads);
@@ -301,7 +300,7 @@ int main()
             Thread::Sleep( 300 );
 
    }
-   
+
    // Tell the threads to shut down, then wait until they all
    // finish.
    Test::running = false;
@@ -310,7 +309,7 @@ int main()
       t[ i ]->Join();
 
    }
-   
+
    // Display statistics.
    Console::WriteLine( "\r\n {0} reads, {1} writes, {2} reader time-outs, {3} writer time-outs.", Test::reads, Test::writes, Test::readerTimeouts, Test::writerTimeouts );
    Console::WriteLine( "Press ENTER to exit." );

samples/snippets/csharp/VS_Snippets_CLR_System/system.Threading.ReaderWriterLock/CS/source.cs

@@ -13,7 +13,6 @@ public class Example
 
    const int numThreads = 26;
    static bool running = true;
-   static Random rnd = new Random();
 
    // Statistics.
    static int readerTimeouts = 0;
@@ -48,18 +47,20 @@ public static void Main()
 
    static void ThreadProc()
    {
+      Random rnd = new Random();
+
       // Randomly select a way for the thread to read and write from the shared
       // resource.
       while (running) {
          double action = rnd.NextDouble();
          if (action < .8)
             ReadFromResource(10);
          else if (action < .81)
-            ReleaseRestore(50);
+            ReleaseRestore(rnd, 50);
          else if (action < .90)
-            UpgradeDowngrade(100);
+            UpgradeDowngrade(rnd, 100);
          else
-            WriteToResource(100);
+            WriteToResource(rnd, 100);
       }
    }
 
@@ -88,7 +89,7 @@ static void ReadFromResource(int timeOut)
 
    //<Snippet4>
    // Request and release the writer lock, and handle time-outs.
-   static void WriteToResource(int timeOut)
+   static void WriteToResource(Random rnd, int timeOut)
    {
       try {
          rwl.AcquireWriterLock(timeOut);
@@ -113,7 +114,7 @@ static void WriteToResource(int timeOut)
    //<Snippet5>
    // Requests a reader lock, upgrades the reader lock to the writer
    // lock, and downgrades it to a reader lock again.
-   static void UpgradeDowngrade(int timeOut)
+   static void UpgradeDowngrade(Random rnd, int timeOut)
    {
       try {
          rwl.AcquireReaderLock(timeOut);
@@ -164,7 +165,7 @@ static void UpgradeDowngrade(int timeOut)
    // Release all locks and later restores the lock state.
    // Uses sequence numbers to determine whether another thread has
    // obtained a writer lock since this thread last accessed the resource.
-   static void ReleaseRestore(int timeOut)
+   static void ReleaseRestore(Random rnd, int timeOut)
    {
       int lastWriter;