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

[Petros Douvantzis]

Hi,

I will file a bug.

I think that I should file one bug int the iOS BCL
<https://bugzilla.xamarin.com/buglist.cgi?component=BCL%20Class%20Libraries&list_id=218333&product=iOS&resolution=--->
libraries
and one for the Android BCL
<https://bugzilla.xamarin.com/buglist.cgi?component=BCL%20Class%20Libraries&list_id=218332&product=Android&resolution=--->,
right? I guess the solution will be related to one another though.

Best,

On Thu, Jul 7, 2016 at 11:20 AM, Alex Rønne Petersen <alex at alexrp.com>
wrote:

> Hi,
>
> It is correct that the volatile keyword should result in
> acquire/release barriers as a result of compiling down to
> Thread.VolatileRead () / VolatileWrite () calls. In theory, the only
> difference between the Thread and Volatile methods is that the
> Volatile methods will actually be atomic for 64-bit quantities on a
> 32-bit machine, where the Thread methods will not (incidentally, this
> is why the volatile keyword is not allowed on 64-bit types). But since
> you're using a 32-bit value, this shouldn't matter. So the fact that
> switching the code to the Volatile methods makes it work is very
> strange indeed.
>
> Could you file a bug with the test case you provided?
>
> 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.
> > _______________________________________________
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> > Mono-devel-list at lists.ximian.com
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>



-- 

Petros Douvantzis

Co-founder

Horizon Video Technologies

horizon.camera
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