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://msdn.microsoft.com/en-us/library/x13ttww7.aspx> 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 <http://csharpindepth.com/Articles/General/Singleton.aspx> ) 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://msdn.microsoft.com/en-us/magazine/jj883956.aspx>. 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 > > > > 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
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