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    <p>This is a (bad) form of the double check locking pattern which is <strong>not</strong> thread safe in C#!</p> <p>There is one big problem in this code:</p> <p>s_Initialized is not volatile. That means that writes in the initialization code can move after s_Initialized is set to true and other threads can see uninitialized code even if s_Initialized is true for them. This doesn't apply to Microsoft's implementation of the Framework because every write is a volatile write.</p> <p>But also in Microsoft's implementation, reads of the uninitialized data can be reordered (i.e. prefetched by the cpu), so if s_Initialized is true, reading the data that should be initialized can result in reading old, uninitialized data because of cache-hits (ie. the reads are reordered).</p> <p>For example: </p> <pre><code>Thread 1 reads s_Provider (which is null) Thread 2 initializes the data Thread 2 sets s\_Initialized to true Thread 1 reads s\_Initialized (which is true now) Thread 1 uses the previously read Provider and gets a NullReferenceException </code></pre> <p>Moving the read of s_Provider before the read of s_Initialized is perfectly legal because there is no volatile read anywhere.</p> <p>If s_Initialized would be volatile, the read of s_Provider would not be allowed to move before the read of s_Initialized and also the initialization of the Provider is not allowed to move after s_Initialized is set to true and everything is ok now.</p> <p>Joe Duffy also wrote an Article about this problem: <a href="http://www.bluebytesoftware.com/blog/PermaLink,guid,3420c247-2da5-411b-8ce7-05082e1aba30.aspx" rel="noreferrer">Broken variants on double-checked locking</a></p>
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    1. POIs accessing a variable in C# an atomic operation?
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    1. USabatishchev
    1. USThomas Danecker
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    1. COGood point. Reading the article you linked to, whether the code in Membership is right or wrong seems to depend on the precise details of the compiler and the processor, but I agree that s_Initialized should be made volatile just to be sure.
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      1. USRory MacLeod
    2. COIt *is* wrong according to .net's Memory Modell. On certain processors and/or with certain compilers, you may be lucky enought that it still works. Joe Duffy wrote another article: http://www.bluebytesoftware.com/blog/2008/07/17/LoadsCannotPassOtherLoadsIsAMyth.aspx
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      1. USThomas Danecker
    3. COThe absence of write reordering is a characteristic of x86 and x64 but not necessarily Microsoft's .NET, i.e. I would not rely upon it if Microsoft add support for other architectures where the hardware does reorder writes.
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      1. USJon Harrop
 

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