[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 13:42:12 UTC 2016
adb shell setprop debug.mono.env "'MONO_ENV_OPTIONS=-O=-intrins'"
(with all of the quotes)
Yes, runtime/JIT section. Thanks!
On Thu, Jul 7, 2016 at 11:20 AM, Petros Douvantzis <petrakeas at gmail.com> wrote:
> I ran:
> 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 section?
> 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>
>> > 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.
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> Petros Douvantzis
> Horizon Video Technologies
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