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

Thu Jul 7 15:38:17 UTC 2016

Hi Petros,

It does look like a bug in our end. We do enforce ECMA's load-acquire,
store-release semantics for volatiles.

Can we integrate your test case into mono?

--
Rodrigo

On Thu, Jul 7, 2016 at 11:05 AM, Petros Douvantzis <petrakeas at gmail.com>
wrote:

> Hello Miguel,
>
> The initial code does *not *have the field marked as volatile. However,
> it may work in Net 2.0 because it has stronger memory guarantees than the
> ECMA.
>
> So, the articles continues saying "*Making the instance variable volatile
> can make it work*". So, *if* the field were volatile, it would work in
> every ECMA implementation.
>
> Also, I tried using:
> adb shell setprop debug.mono.env "'MONO_ENV_OPTIONS=-O=-intrins'"
> with no difference in the outcome.
>
> Best,
>
> On Thu, Jul 7, 2016 at 5:59 PM, Miguel de Icaza <miguel at microsoft.com>
> wrote:
>
>> Hello Petros,
>>
>>
>>
>> That blog post on double-check-locking explicitly states that without
>> extra steps the pattern would not work.   Maybe I missed something?
>>
>>
>>
>> From that post:
>>
>> ·         Without any memory barriers, it's broken in the ECMA CLI
>> specification too. It's possible that under the .NET 2.0 memory model
>> (which is stronger than the ECMA spec) it's safe, but I'd rather not rely
>> on those stronger semantics, especially if there's any doubt as to the
>> safety. Making the instance variable volatile can make it work, as would
>> explicit memory barrier calls, although in the latter case even experts
>> can't agree exactly which barriers are required. I tend to try to avoid
>> situations where experts don't agree what's right and what's wrong!
>>
>>
>>
>>
>>
>>
>>
>> *From: *Petros Douvantzis <petrakeas at gmail.com>
>> *Date: *Thursday, July 7, 2016 at 3:54 AM
>> *To: *"mono-devel-list at lists.ximian.com" <
>> mono-devel-list at lists.ximian.com>, Miguel de Icaza <miguel at microsoft.com>
>> *Cc: *Rodrigo Kumpera <kumpera at gmail.com>
>>
>> *Subject: *Re: [Mono-dev] Volatile fields don't enforce acquire -
>> release semantics like Volatile.Read() and Volatile.Write()
>>
>>
>>
>> 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://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2fx13ttww7.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=s02wpedE5%2fUhB9yawFzf3QlY51QePjK5rUj1c16Selk%3d>
>>  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
>> <https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fcsharpindepth.com%2fArticles%2fGeneral%2fSingleton.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=yk%2bj4W775WX%2b82pOGWXA4xyURhfDzV1XSvJle2p3L2w%3d> )
>> 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://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2fmagazine%2fjj883956.aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=Edq89e1H1sGysBYfBQFrb9WUTXczZe0ZlQfh1FQJvJc%3d>
>> .
>>
>>
>>
>> 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 <miguel at 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
>> <https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fmsdn.microsoft.com%2fen-us%2flibrary%2fgg712971(v%3dvs.110).aspx&data=01%7c01%7cmiguel%40microsoft.com%7cbdcbcb26fc8744e9b67d08d3a63c01c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=U4RIuTFZa6oqRlI3uSyP2q8by90V0mpKDfgmnqQNUZI%3d>
>>
>>
>>
>> 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-bounces at lists.ximian.com> on behalf of Rodrigo
>> Kumpera <kumpera at gmail.com>
>> *Date: *Wednesday, July 6, 2016 at 1:27 PM
>> *To: *petrakeas <petrakeas at gmail.com>
>> *Cc: *"mono-devel-list at lists.ximian.com" <
>> mono-devel-list at 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 <petrakeas at 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.
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>>
>>
>>
>>
>>
>>
>>
>> --
>>
>> Petros Douvantzis
>>
>> Co-founder
>>
>> Horizon Video Technologies
>>
>> *horizon.camera
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>>
>>
>>
>>
>
>
> --
>
> Petros Douvantzis
>
> Co-founder
>
> Horizon Video Technologies
>
> horizon.camera
>
>
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