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

[Petros Douvantzis]

I ran:
adb shell setprop debug.mono.env "-O=-intrins"

Is this correct?

There was no difference in the outcome: When volatile keyword is used,
errors occur. When Volatile class is used, no errors were spotted.

Should I file a bug to the Runtime/JIT
<https://bugzilla.xamarin.com/buglist.cgi?component=JIT&list_id=218467&product=Runtime&resolution=--->
 section?

Where

On Thu, Jul 7, 2016 at 11:32 AM, Alex Rønne Petersen <alex at alexrp.com>
wrote:

> 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?
> >
> >
> >
> >
> > --
> > View this message in context:
> http://mono.1490590.n4.nabble.com/Volatile-fields-don-t-enforce-acquire-release-semantics-like-Volatile-Read-and-Volatile-Write-tp4668111.html
> > Sent from the Mono - Dev mailing list archive at Nabble.com.
> > _______________________________________________
> > Mono-devel-list mailing list
> > Mono-devel-list at lists.ximian.com
> > http://lists.ximian.com/mailman/listinfo/mono-devel-list
>



-- 

Petros Douvantzis

Co-founder

Horizon Video Technologies

horizon.camera
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ximian.com/pipermail/mono-devel-list/attachments/20160707/7f823307/attachment.html>