Memory barrier generators

Solution 1:

Here is my take on the subject and to attempt to provide a quasi-complete list in one answer. If I run across any others I will edit my answer from time to time.

Mechanisms that are generally agreed upon to cause implicit barriers:

• All Monitor class methods including the C# keyword lock
• All Interlocked class methods.
• All Volatile class methods (.NET 4.5+).
• Most SpinLock methods including Enter and Exit.
• Thread.Join
• Thread.VolatileRead and Thread.VolatileWrite
• Thread.MemoryBarrier
• The volatile keyword.
• Anything that starts a thread or causes a delegate to execute on another thread including QueueUserWorkItem, Task.Factory.StartNew, Thread.Start, compiler supplied BeginInvoke methods, etc.
• Using a signaling mechanism such as ManualResetEvent, AutoResetEvent, CountdownEvent, Semaphore, Barrier, etc.
• Using marshaling operations such as Control.Invoke, Dispatcher.Invoke, SynchronizationContext.Post, etc.

Mechanisms that are speculated (but not known for certain) to cause implicit barriers:

• Thread.Sleep (proposed by myself and possibly others due to the fact that code which exhibits a memory barrier problem can be fixed with this method)
• Thread.Yield
• Thread.SpinWait
• Lazy<T> depending on which LazyThreadSafetyMode is specified

Other notable mentions:

• Default add and remove handlers for events in C# since they use lock or Interlocked.CompareExchange.
• x86 stores have release fence semantics
• Microsoft's implemenation of the CLI has release fence semantics on writes despite the fact that the ECMA specification does not mandate it.
• MarshalByRefObject seems to suppress certain optimizations in subclasses which may make it appear as if an implicit memory barrier were present. Thanks to Hans Passant for discovering this and bringing it to my attention.¹

¹This explains why BackgroundWorker works correctly without having volatile on the underlying field for the CancellationPending property.

Solution 2:

I seem to recall that the implementations of the Thread.VolatileRead and Thread.VolatileWrite methods actually cause full fences, not half fences.

This is deeply unfortunate, as people might have come to rely upon this behaviour unknowingly; they might have written a program that requires a full fence, think they need a half fence, think they are getting a half fence, and will be in for a nasty surprise if an implementation of these methods ever does provide a half fence.

I would avoid these methods. Of course, I would avoid everything involving low-lock code, not being smart enough to write it correctly in anything but the most trivial cases.