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

Thu Jul 7 15:05:56 UTC 2016

Hello Miguel,

The initial code does *not *have the field marked as volatile. However, it
may work in Net 2.0 because it has stronger memory guarantees than the
ECMA.

So, the articles continues saying "*Making the instance variable volatile
can make it work*". So, *if* the field were volatile, it would work in
every ECMA implementation.

Also, I tried using:
adb shell setprop debug.mono.env "'MONO_ENV_OPTIONS=-O=-intrins'"
with no difference in the outcome.

Best,

On Thu, Jul 7, 2016 at 5:59 PM, Miguel de Icaza <miguel at microsoft.com>
wrote:

> Hello Petros,
>
>
>
> That blog post on double-check-locking explicitly states that without
> extra steps the pattern would not work.   Maybe I missed something?
>
>
>
> From that post:
>
> ·         Without any memory barriers, it's broken in the ECMA CLI
> specification too. It's possible that under the .NET 2.0 memory model
> (which is stronger than the ECMA spec) it's safe, but I'd rather not rely
> on those stronger semantics, especially if there's any doubt as to the
> safety. Making the instance variable volatile can make it work, as would
> explicit memory barrier calls, although in the latter case even experts
> can't agree exactly which barriers are required. I tend to try to avoid
> situations where experts don't agree what's right and what's wrong!
>
>
>
>
>
>
>
> *From: *Petros Douvantzis <petrakeas at gmail.com>
> *Date: *Thursday, July 7, 2016 at 3:54 AM
> *To: *"mono-devel-list at lists.ximian.com" <mono-devel-list at lists.ximian.com>,
> Miguel de Icaza <miguel at microsoft.com>
> *Cc: *Rodrigo Kumpera <kumpera at gmail.com>
>
> *Subject: *Re: [Mono-dev] Volatile fields don't enforce acquire - release
> semantics like Volatile.Read() and Volatile.Write()
>
>
>
> Hello Miguel,
>
>
>
> I see your point. Even worse, it's a bit ambiguous of what guarantees does
> the volatile field make. For example in MSDN
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2fx13ttww7.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=s02wpedE5%2fUhB9yawFzf3QlY51QePjK5rUj1c16Selk%3d>
>  nothing is mentioned about fences or memory re-orders. In that sense, Mono
> is correct. However, in ECMA 335 they mention acquire-release semantics
> such as the ones in the Volatile class you mentioned.
>
>
>
> One this to consider is that if the Volatile field does not have these
> semantics, then* lazy initialization* that relies on a volatile field and
> a lock ( double-check locking
> <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fcsharpindepth.com%2fArticles%2fGeneral%2fSingleton.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=yk%2bj4W775WX%2b82pOGWXA4xyURhfDzV1XSvJle2p3L2w%3d> )
> is *broken *in Mono for iOS and Android, because there is a chance that a
> half created object is viewed by another thread. The way the volatile field
> is supposed to help, is explained in this post
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2fmagazine%2fjj883956.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=Edq89e1H1sGysBYfBQFrb9WUTXczZe0ZlQfh1FQJvJc%3d>
> .
>
>
>
> The only way to make this work right now is using the Volatile class, but
> most probably someone would expect the volatile field to work.
>
>
>
> Best,
>
>
>
> On Wed, Jul 6, 2016 at 9:24 PM, Miguel de Icaza <miguel at microsoft.com>
> wrote:
>
> Hello,
>
>
>
> I am not convinced that this is a bug worth fixing.
>
>
>
> I think this requires some thinking.  While this might have been the
> intended visible behavior from C#, this predates the extensive use of C#
> beyond the x86 platform.   I believe this is why the Volatile APIs were
> introduced.
>
>
>
> Consder the documentation for it:
>
>
>
> https://msdn.microsoft.com/en-us/library/gg712971(v=vs.110).aspx
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2fgg712971(v%3dvs.110).aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=U4RIuTFZa6oqRlI3uSyP2q8by90V0mpKDfgmnqQNUZI%3d>
>
>
>
> If the language/runtime already provided this support, there would be no
> need for these volatile methods in the first place.
>
>
>
> Miguel.
>
>
>
> *From: *<mono-devel-list-bounces at lists.ximian.com> on behalf of Rodrigo
> Kumpera <kumpera at gmail.com>
> *Date: *Wednesday, July 6, 2016 at 1:27 PM
> *To: *petrakeas <petrakeas at gmail.com>
> *Cc: *"mono-devel-list at lists.ximian.com" <mono-devel-list at lists.ximian.com
> >
> *Subject: *Re: [Mono-dev] Volatile fields don't enforce acquire - release
> semantics like Volatile.Read() and Volatile.Write()
>
>
>
> Yes, it looks like a bug.
>
>
>
> On Wed, Jul 6, 2016 at 11:13 AM, petrakeas <petrakeas at gmail.com> wrote:
>
> According to C#  specification
> <https://msdn.microsoft.com/en-us/library/ms228593.aspx
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2fms228593.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cf3c960accdeb43d8500208d3a5c2d9ae%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=66AJc2tU2gcy4vutTkC%2b4bPl3MxAnAiXd4POGNZ3ivA%3d>>
> :
>
> •       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
> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2faa645755(v%3dvs.71).aspx&data=01%7c01%7cmiguel%40microsoft.com%7cf3c960accdeb43d8500208d3a5c2d9ae%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=cFpmsRLE5a248vj3svbpsmOBouE%2bOxE%2fwDMWjd0YkhU%3d>>
>  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
> <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmono.1490590.n4.nabble.com%2ffile%2fn4668111%2fProgram.cs&data=01%7c01%7cmiguel%40microsoft.com%7cf3c960accdeb43d8500208d3a5c2d9ae%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=H7V6%2bpq4jV8Kw7QdgZMVJRav%2b1XovSCuIY3PgRFaJrk%3d>>
> .
>
> 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:
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>
>
>
>
>
>
> --
>
> Petros Douvantzis
>
> Co-founder
>
> Horizon Video Technologies
>
> *horizon.camera
> <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fhorizon.camera&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=k%2bCLVnzGEb%2fX6zRRD4SfroHMqrvOcV7WJaGEOt2KYqM%3d>*
>
>
>
>

-- 

Petros Douvantzis

Co-founder

Horizon Video Technologies

horizon.camera
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