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[Xen-devel] Re: VM hung after running sometime

To: MaoXiaoyun <tinnycloud@xxxxxxxxxxx>
Subject: [Xen-devel] Re: VM hung after running sometime
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 20 Sep 2010 08:45:21 +0100
Cc: xen devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On 20/09/2010 07:00, "MaoXiaoyun" <tinnycloud@xxxxxxxxxxx> wrote:

> When IO is not ready, domain U in VMEXIT->hvm_do_resume might invoke
> wait_on_xen_event_channel
> (where it is blocked in _VPF_blocked_in_xen).
> Here is my assumption of event missed.
> step 1: hvm_do_resume execute 260, and suppose p->state is STATE_IOREQ_READY
> step 2: then in cpu_handle_ioreq is in line 547, it execute line 548 so
> quickly before hvm_do_resume execute line 270.
> Well, the event is missed.
> In other words, the _VPF_blocked_in_xen is cleared before it is actually
> setted, and Domian U who is blocked
> might never get unblocked, it this possible?

Firstly, that code is very paranoid and it should never actually be the case
that we see STATE_IOREQ_READY or STATE_IOREQ_INPROCESS in hvm_do_resume().
Secondly, even if you do, take a look at the implementation of
wait_on_xen_event_channel() -- it is smart enough to avoid the race you

 -- Keir

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