Re: [Xen-devel] [PATCH 2/2] xenbus: bypass xenbus frontend resume if xenstored is not running

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.05.13 at 10:24, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Wed, 2013-05-01 at 13:57 +0100, Aurelien Chartier wrote:
>> If the xenbus frontend is running in a domain running xenstored or in dom0,
>> the device resume is hanging because it is happening before the process
>> resume. This patch adds extra logic to the resume code to check if we are
>> the domain running xenstored or dom0.
>> 
>> The frontend will be reconnected later, when the backend resumes from S3.
>> This logic is working when xenstored is running in dom0, but has not been
>> tested with a xenstore stub domain.
>> ---
>>  drivers/xen/xenbus/xenbus_probe_frontend.c |   15 ++++++++++++++-
>>  1 file changed, 14 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/xen/xenbus/xenbus_probe_frontend.c 
> b/drivers/xen/xenbus/xenbus_probe_frontend.c
>> index 3159a37..8583afe 100644
>> --- a/drivers/xen/xenbus/xenbus_probe_frontend.c
>> +++ b/drivers/xen/xenbus/xenbus_probe_frontend.c
>> @@ -89,9 +89,22 @@ static void backend_changed(struct xenbus_watch *watch,
>>      xenbus_otherend_changed(watch, vec, len, 1);
>>  }
>>  
>> +static int xenbus_frontend_dev_resume(struct device *dev)
>> +{
>> +    /* 
>> +     * If xenstored is running in that domain, we cannot access the backend
>> +     * state at the moment. If we are running in dom0, the domain running
>> +     * xenstored is still suspended at that point
>> +     */
>> +    if (xen_initial_domain() || (xen_store_domain == XS_LOCAL))
>> +            return 0;
>> +
>> +    return xenbus_dev_resume(dev);
> 
> When or where does this eventually get called for the init domain or
> XS_LOCAL cases?

I was about to ask the same question. Plus I don't think the
description here or in the overview mail really makes clear how
specifically a deadlock would occur here. That's pretty relevant to
understand in the light that so far we had no indication of there
being any special treatment necessary here, and resume from S3
had been working quite fine without that (at least as long as
xenstored is running in Dom0 and at least with the traditional/
forward-port/non-pvops kernels).

Jan


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