[jira] [Comment Edited] (THRIFT-3038) Use of volatile in cpp library

["Tomasz Kupczyk (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/THRIFT-3038?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14561759#comment-14561759 ] 

Tomasz Kupczyk edited comment on THRIFT-3038 at 5/27/15 9:20 PM:
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For issue number 1. it looks like it would be enough to make `writerWaiting_` an atomic integer/bool variable. That would solve the synchronization problem and the data race. I haven't noticed though usage of any atomic types in thrift cpp files. 'C++11' supports atomic types and as far as I can see we compile with '-std=c++11' option. It can have an impact on performance as as far as I'm aware when using atomics memory barriers may are involved. It's hard to say what will be the impact. I can provide a patch for that and try do some performance testing. Any objections/suggestions?


was (Author: flojdek):
For issue number 1. it looks like it would be enough to make `writerWaiting_` an atomic integer/bool variable. That would solve the synchronization problem and the data race. I haven't noticed though usage of any atomic types in thrift cpp files. C++11 supports atomic types and as far as I can see we compile with -std=c++11 option. It can have an impact on performance as as far as I'm aware when using atomics memory barriers may are involved. It's hard to say what will be the impact. I can provide a patch for that and try do some performance testing. Any objections/suggestions?

> Use of volatile in cpp library
> ------------------------------
>
>                 Key: THRIFT-3038
>                 URL: https://issues.apache.org/jira/browse/THRIFT-3038
>             Project: Thrift
>          Issue Type: Bug
>          Components: C++ - Library
>    Affects Versions: 0.9.2
>            Reporter: Adriaan Schmidt
>
> In the cpp library there are several member variables which are declared volatile, I believe with the intention of providing some sort of thread-safety. 
> While volatile can be used in this way in Java and C#, in C++ it cannot! It does not provide any guarantees with regard to instruction (re-)ordering, i.e. there are no implied memory barriers like you would get by using explicit locking or atomic variables.
> This means that all uses of volatile should be examined, the volatile qualifier should be removed and replaced by proper synchronization.
> The affected member variables are:
> # NoStarveReadWriteMutex::writerWaiting_
> Unprotected read access in acquireRead(). Data race can be seen by running the unit test with Helgrind.
> # TFileTransport::forceFlush_
> Always accessed while holding mutex_. In this case, the volatile can just be removed.
> # TFileTransport::closing_
> Sometimes accessed while holding mutex_ (in combination with the notEmpty_ Monitor),
> but, e.g., enqueueEvent reads closing_ without any synchronization.
> # TThreadPoolServer::stop_, TThreadedServer::stop_
> Accessed (read and written) without synchronization. These would probably be fine using an atomic data type. Or, use explicit locking or signaling. 
> # TThreadPoolServer::timeout_, TThreadPoolServer::taskExpiration_
> Should probably use a lock.
> # Mutex.cpp has mutexProfilingCounter as static variable. This probably doesn’t break anything, but still the volatile serves no real purpose.
> While some of the fixes are probably simple, in general I think someone with better knowledge of the code should have a look at this.



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