[libvirt] [PATCH 2/2] Fix updating of haveTheBuck in RPC client to be race-free

Peter Krempa pkrempa at redhat.com
Wed Dec 7 15:29:02 UTC 2011

On 12/07/2011 03:46 PM, Daniel P. Berrange wrote:
> From: "Daniel P. Berrange"<berrange at redhat.com>
> When one thread passes the buck to another thread, it uses
> virCondSignal to wake up the target thread. The variable
> 'haveTheBuck' is not updated in a race-free manner when
> this occurs. The current thread sets it to false, and the
> woken up thread sets it to true. There is a window where
> a 3rd thread can come in and grab the buck.
> Even if this didn't lead to crashes&  deadlocks, this would
> still result in unfairness in the buckpassing algorithm.
> A better solution is to *never* set haveTheBuck to false
> when we're passing the buck. Only set it to false when there
> is no further thread waiting for the buck.
> * src/rpc/virnetclient.c: Only set haveTheBuck to false
>    if no thread is waiting
> ---
I've tested it with virt-manager and also the trheaded test. Works fine 
to me and looks good.



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