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Date: Thu, 16 May 2013 09:30:46 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 2/8] block: add basic backup support to
 block driver
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To: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org, imain@redhat.com, Paolo Bonzini <pbonzini@redhat.com>, dietmar@proxmox.com

On Thu, May 16, 2013 at 11:27:38AM +0800, Wenchao Xia wrote:
> > +/* See if in-flight requests overlap and wait for them to complete */
> > +static void coroutine_fn wait_for_overlapping_requests(BackupBlockJob *job,
> > +                                                       int64_t start,
> > +                                                       int64_t end)
> > +{
> > +    CowRequest *req;
> > +    bool retry;
> > +
> > +    do {
> > +        retry = false;
> > +        QLIST_FOREACH(req, &job->inflight_reqs, list) {
> > +            if (end > req->start && start < req->end) {
> > +                qemu_co_queue_wait(&req->wait_queue);
> > +                retry = true;
> > +                break;
> > +            }
> > +        }
> > +    } while (retry);
> > +}
> > +
> 
>   In my understanding, there will be possible two program routines entering
> here at same time since it holds read lock instead of write lock, and
> they may also modify job->inflight_reqs, is it possible a race
> condition here? I am not sure whether back-ground job will becomes a
> thread.

No, all operations on a BlockDriverState execute in the same event loop
thread.  Only coroutine synchronization is necessary, which is provided
in these patches.