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

Alex Rønne Petersen alex at alexrp.com
Thu Jul 7 08:20:21 UTC 2016


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?


On Wed, Jul 6, 2016 at 5:13 PM, petrakeas <petrakeas at 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?
> --
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