Re: [Qemu-devel] [PATCH v2 19/24] block: Allow wait_serialising_requests() at any point

[Wenchao Xia]

于 2014/1/13 19:29, Kevin Wolf 写道:
> Am 27.12.2013 um 05:17 hat Wenchao Xia geschrieben:
> > 于 2013/12/13 21:22, Kevin Wolf 写道:
> > > We can only have a single wait_serialising_requests() call per request
> > > because otherwise we can run into deadlocks where requests are waiting
> > > for each other.
> >    do you mean:
> > mark_request_serialising(req)
> > ...
> > wait_serialising_requests(req);
> > ...
> > wait_serialising_requests(req);
> >
> >   will have deadlock?
>
> Yes, it can deadlock (it doesn't have to, it depends on whether another
> overlapping request is started concurrently). More precisely, the
> problematic pattern is:
>
>      mark_request_serialising(req);
>      ...
>      qemu_coroutine_yield(); /* Other requests may be issued now */
>      ...
>      wait_serialising_requests(req);
>
> What you mentioned above is a special case of this.

  It seems when two overlapping request exist in the qeueue, and
both are waiting, problem happens. Thanks for explanation, I am fine
with this patch.


> >   I thought it is already resolved by patch 15?
> > Maybe here is another deadlock reason?
>
> The problematic pattern in patch 15 was:
>
>      mark_request_serialising(req);
>      ... /* no yield here */
>      wait_serialising_requests(req);
>
> As opposed to the originally used:
>
>      wait_serialising_requests(req);
>      ... /* no yield here */
>      mark_request_serialising(req);
>
> Kevin