Bug 42413

Summary: Volatile fields don't enforce acquire - release semantics like Volatile.Read() and Volatile.Write()
Product: [Mono] Runtime Reporter: Petros Douvantzis <petrakeas>
Component: JITAssignee: Bugzilla <bugzilla>
Status: RESOLVED FIXED    
Severity: normal CC: mono-bugs+mono, mono-bugs+runtime, petrakeas, vargaz
Priority: ---    
Version: 4.4.0 (C7)   
Target Milestone: ---   
Hardware: All   
OS: All   
Tags: Is this bug a regression?: ---
Last known good build:
Attachments: 2 test scripts that reproduce the error in iOS and Android

Description Petros Douvantzis 2016-07-07 22:39:44 UTC
Created attachment 16607 [details]
2 test scripts that reproduce the error in iOS and Android

According to C# specification: 

•	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.

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 am attaching a zip that contains 2 tests that reproduce the problem in iOS and Android. I have added more details in the readme file.

The issue has been discussed in the dev list: http://mono.1490590.n4.nabble.com/Volatile-fields-don-t-enforce-acquire-release-semantics-like-Volatile-Read-and-Volatile-Write-td4668111.html
Comment 1 Zoltan Varga 2016-07-08 00:15:24 UTC
Fixed in mono master/4.6.0 branch. Thanks for the testcase.
Comment 2 Petros Douvantzis 2016-07-08 07:40:04 UTC
That was fast!
Thank you!