[libvirt] Potential race condition problem

[Benjamin Wang (gendwang)]

Hi,
I think you misunderstand my meaning. My solution includes step1 + step2. Step1 is used to implement thread mutex. Step2 is used to
handle “initialized” visibility. Without step2, the initialization could be executed several times.

B.R.
Benjamin Wang

From: Guannan Ren [mailto:gren at redhat.com]
Sent: 2012年9月29日 17:22
To: Benjamin Wang (gendwang)
Cc: Daniel Veillard; libvir-list at redhat.com; Yang Zhou (yangzho); cbley at av-test.de
Subject: Re: [libvirt] Potential race condition problem

On 09/29/2012 03:52 PM, Benjamin Wang (gendwang) wrote:
Hi,
OK. Now I am using JNA to access libvirt. If we add another mutex which used to access “initialized” parameter. This mutex must be pthread_mutex_init firstly and only once.
But it seems that there is no way to change libvirt code. I do it as following:

1.      Changing libvirt JNA code in Connect.java
Old Code:
    public Connect(String uri) throws LibvirtException {
         VCP = libvirt.virConnectOpen(uri);
        // Check for an error
        processError();
        ErrorHandler.processError(Libvirt.INSTANCE);
    }

New Code:
    public Connect(String uri) throws LibvirtException {
         synchronized(this.getClass()) {
                  VCP = libvirt.virConnectOpen(uri);
         }
        // Check for an error
        processError();
        ErrorHandler.processError(Libvirt.INSTANCE);
    }

This can make sure only that one thread can execute Connect. For a server application, we only need one time. So the performance is OK


2.      Changing libvirt code in libvirt.c
Old Code:
static int initialized = 0;

New Code:
static int volatile initialized = 0;

This can make sure the initialization will be executed once.

Would you give your comments for this solution?

B.R.
Benjamin Wang


     As far as I know the operations on volatile variable is not atomic,
  the usage of volatile keyword as a portable synchronization mechanism is discouraged by C.
     But in Java, it is a global ordering on the reads and writes to a volatile variable.
  So, maybe, your first solution is pretty enough good.

  Guannan

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