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

[Alex Rønne Petersen]

Hi,

By the way, I would suggest trying to run the app with something like:

    MONO_ENV_OPTIONS="-O=-intrins"

For Android, see here how to set this:
https://developer.xamarin.com/guides/android/advanced_topics/environment/

For iOS, you'd need to set this when invoking the AOT compiler. I'm
not really familiar with where you'd need to do this, though.

This would disable the JIT's intrinsics for the various atomics /
memory model methods in the framework. It would be good to know if
this makes the test case work or if the result is the same, as we
could narrow the problem down to either the JIT's intrinsics or the
fallback C code in the runtime.

Regards,
Alex

On Wed, Jul 6, 2016 at 5:13 PM, petrakeas <petrakeas at gmail.com> wrote:
> 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
> sequence.
>
> 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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