[Mono-dev] Volatile fields don't enforce acquire - release semantics like Volatile.Read() and Volatile.Write()

petrakeas petrakeas at gmail.com
Wed Jul 6 15:13:25 UTC 2016

According to C#  specification
<https://msdn.microsoft.com/en-us/library/ms228593.aspx>  :

•	A read of a volatile field is called a volatile read. A volatile read has
“acquire semantics”; that is, it is guaranteed to occur prior to any
references to memory that occur after it in the instruction sequence. 
•	A write of a volatile field is called a volatile write. A volatile write
has “release semantics”; that is, it is guaranteed to happen after any
memory references prior to the write instruction in the instruction

The spec presents  an example
<https://msdn.microsoft.com/en-us/library/aa645755(v=vs.71).aspx>   where
one thread writes "data" on a non volatile variable and "publishes" the
result by writing on a volatile variable that acts as a flag. The other
thread checks the volatile flag and if set, it accesses the non-volatile
variable that is now *guaranteed* to contain the data.

It seems that Mono 4.4 (the one used in Xamarin) does not enforce these
semantics or in other words does not prevent memory re-ordering in Android
and iOS that have relaxed memory models due to their CPU.

I have created an a test that reproduces the problem in iOS and Android 
Program.cs <http://mono.1490590.n4.nabble.com/file/n4668111/Program.cs>  . 

If the access to the volatile field is replaced by Volatile.Read() and
Volatile.Write(), then no-problems occur. It seems that Volatile.Read() and
Volatile.Write() implement half fences in Mono, but the volatile keyword
does not. 

Is this a bug?

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