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 <petrak...@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. > _______________________________________________ > Mono-devel-list mailing list > Mono-devel-list@lists.ximian.com > http://lists.ximian.com/mailman/listinfo/mono-devel-list _______________________________________________ Mono-devel-list mailing list Mono-devel-list@lists.ximian.com http://lists.ximian.com/mailman/listinfo/mono-devel-list
