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
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