[Mono-dev] Volatile fields don't enforce acquire - release semantics like Volatile.Read() and Volatile.Write()
petrakeas at gmail.com
Thu Jul 7 09:20:38 UTC 2016
adb shell setprop debug.mono.env "-O=-intrins"
Is this correct?
There was no difference in the outcome: When volatile keyword is used,
errors occur. When Volatile class is used, no errors were spotted.
Should I file a bug to the Runtime/JIT
On Thu, Jul 7, 2016 at 11:32 AM, Alex Rønne Petersen <alex at alexrp.com>
> By the way, I would suggest trying to run the app with something like:
> For Android, see here how to set this:
> For iOS, you'd need to set this when invoking the AOT compiler. I'm
> not really familiar with where you'd need to do this, though.
> This would disable the JIT's intrinsics for the various atomics /
> memory model methods in the framework. It would be good to know if
> this makes the test case work or if the result is the same, as we
> could narrow the problem down to either the JIT's intrinsics or the
> fallback C code in the runtime.
> 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>
> > 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
> > 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()
> > Volatile.Write() implement half fences in Mono, but the volatile keyword
> > does not.
> > Is this a bug?
> > --
> > View this message in context:
> > Sent from the Mono - Dev mailing list archive at Nabble.com.
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