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POreaderwriterlock allowing reads while write lock is acquired?
 

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  1. POreaderwriterlock allowing reads while write lock is acquired?
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    <p>I have a static class which is accessed by multiple remoting and other internal to the application threads. Part of the functionality of this class is controlling read/write access to various files, so I've implemented a static ReaderWriterLock on the list of files. The project uses the .net framework 2.0 as part of the customer requirements.</p> <p>However when I stress test the system using a number of different clients (generally I'm using 16) each performing a large amount of reads and writes then very intermittently and only after several hours or even days have passed with at least 500k+ transactions completed the system crashes. Ok so we got a bug..</p> <p>But when I check the logs of all locking events I can see that the following has happened:</p> <p>1: Thread A acquires a write lock directly, checking IsWriterLock shows it to be true.</p> <p>2: Thread B tries to acquire a reader lock and <strong>succeeds</strong> even though Thread A still has the write lock</p> <p>3: System now crashes, stack trace now shows a null reference exception to the readerwriterlock</p> <p>This process has been run several hundred thousand times previously with no errors and I can check the logs and see that the read lock was blocked in all cases previously until the write had exited. I have also tried implementing the readerwriterlock as a singleton but the issue still occurs</p> <p>Has anybody ever seen anything like this before ??</p> <p>A slimed down version of the readerwriterlock implementation used is shown below:</p> <pre><code>private const int readwriterlocktimeoutms = 5000; private static ReaderWriterLock readerWriterLock = new ReaderWriterLock(); // this method will be called by thread A public static void MethodA() { // bool to indicate that we have the lock bool IsTaken = false; try { // get the lock readerWriterLock.AcquireWriterLock(readwriterlocktimeoutms); // log that we have the lock for debug // Logger.LogInfo("MethodA: acquired write lock; writer lock held {0}; reader lock held {1}", readerWriterLock.IsWriterLockHeld.ToString(),readerWriterLock.IsReaderLockHeld.ToString(), ); // mark that we have taken the lock IsTaken = true; } catch(Exception e) { throw new Exception(string.Format("Error getting lock {0} {1}", e.Message, Environment.StackTrace)); } try { // do some work } finally { if (IsTaken) { readerWriterLock.ReleaseWriterLock(); } } } // this method will be called by thread B public static void MethodB() { // bool to indicate that we have the lock bool IsTaken = false; try { // get the lock readerWriterLock.AcquireReaderLock(readwriterlocktimeoutms); // log that we have the lock for debug // Logger.LogInfo("MethodB: acquired read lock; writer lock held {0}; reader lock held {1}", readerWriterLock.IsWriterLockHeld.ToString(),readerWriterLock.IsReaderLockHeld.ToString(), ); // mark that we have taken the lock IsTaken = true; } catch (Exception e) { throw new Exception(string.Format("Error getting lock {0} {1}", e.Message, Environment.StackTrace)); } try { // do some work } finally { if (IsTaken) { readerWriterLock.ReleaseReaderLock(); } } } enter code here </code></pre>
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    1. USJohn Saunders
    1. USJohnv2020
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      1. VTUpMod
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    1. COTo be complete, can you add the IsTaken=true statements?
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      1. POreaderwriterlock allowing reads while write lock is acquired?
      1. USHenk Holterman
    2. COapologies Henk - done now
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      1. USJohnv2020
    3. COI don't know if it explains things, but that code is plain wrong; the release ***must*** be part of the first try/finally - otherwise any exception in the actual work code will result in the lock not being released.
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      1. POreaderwriterlock allowing reads while write lock is acquired?
      1. USMarc Gravell
 

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