A better test case? Awesome! Thanks for looking into this. We're aware of the issue so it's up to you on filing a bug.
On Thu, Jul 7, 2016 at 12:53 PM, Petros Douvantzis <petrak...@gmail.com> wrote: > Hello Rodrigo, > > Sure you can. However this specific test does not reproduce the error in > iOS most of the time. I will send another one that has more repetitions > (and wraps-around the array). > > I should file the bug, right? > > Best, > > On Thu, Jul 7, 2016 at 6:38 PM, Rodrigo Kumpera <kump...@gmail.com> wrote: > >> Hi Petros, >> >> It does look like a bug in our end. We do enforce ECMA's load-acquire, >> store-release semantics for volatiles. >> >> Can we integrate your test case into mono? >> >> >> -- >> Rodrigo >> >> On Thu, Jul 7, 2016 at 11:05 AM, Petros Douvantzis <petrak...@gmail.com> >> wrote: >> >>> 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 <mig...@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 <petrak...@gmail.com> >>>> *Date: *Thursday, July 7, 2016 at 3:54 AM >>>> *To: *"mono-devel-list@lists.ximian.com" < >>>> mono-devel-list@lists.ximian.com>, Miguel de Icaza < >>>> mig...@microsoft.com> >>>> *Cc: *Rodrigo Kumpera <kump...@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 <mig...@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-boun...@lists.ximian.com> on behalf of >>>> Rodrigo Kumpera <kump...@gmail.com> >>>> *Date: *Wednesday, July 6, 2016 at 1:27 PM >>>> *To: *petrakeas <petrak...@gmail.com> >>>> *Cc: *"mono-devel-list@lists.ximian.com" < >>>> mono-devel-list@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 <petrak...@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: >>>> http://mono.1490590.n4.nabble.com/Volatile-fields-don-t-enforce-acquire-release-semantics-like-Volatile-Read-and-Volatile-Write-tp4668111.html >>>> <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmono.1490590.n4.nabble.com%2fVolatile-fields-don-t-enforce-acquire-release-semantics-like-Volatile-Read-and-Volatile-Write-tp4668111.html&data=01%7c01%7cmiguel%40microsoft.com%7cf3c960accdeb43d8500208d3a5c2d9ae%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=sqJVi9saxf7EEGpn6Wpf%2bFEeZX5yCpzK4%2b38x670OEw%3d> >>>> Sent from the Mono - Dev mailing list archive at Nabble.com. >>>> _______________________________________________ >>>> Mono-devel-list mailing list >>>> Mono-devel-list@lists.ximian.com >>>> http://lists.ximian.com/mailman/listinfo/mono-devel-list >>>> <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2flists.ximian.com%2fmailman%2flistinfo%2fmono-devel-list&data=01%7c01%7cmiguel%40microsoft.com%7cf3c960accdeb43d8500208d3a5c2d9ae%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=Sb1mXUpzvfBCP0RJh%2bB2orCGoBH3eV8Z8CY10t1NbC0%3d> >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> -- >>>> >>>> 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 >>> >>> >> > > > -- > > Petros Douvantzis > > Co-founder > > Horizon Video Technologies > > horizon.camera > >
_______________________________________________ Mono-devel-list mailing list Mono-devel-list@lists.ximian.com http://lists.ximian.com/mailman/listinfo/mono-devel-list
