linux-kernel - Re: [Xen-devel] [PATCH 1/2] xenbus: don't bail early from xenbus_dev_request_and

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Date:	Thu, 07 Jul 2016 07:35:24 -0600
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>
Cc:	<xen-devel@...ts.xenproject.org>, <boris.ostrovsky@...cle.com>,
	"Juergen Gross" <JGross@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/2] xenbus: don't bail early from
 xenbus_dev_request_and_reply()

>>> On 07.07.16 at 15:22, <david.vrabel@...rix.com> wrote:
> On 07/07/16 14:13, David Vrabel wrote:
>> On 07/07/16 13:23, Jan Beulich wrote:
>>>>>> On 07.07.16 at 14:17, <david.vrabel@...rix.com> wrote:
>>>> On 07/07/16 13:09, Jan Beulich wrote:
>>>>>>>> On 07.07.16 at 13:36, <david.vrabel@...rix.com> wrote:
>>>>>> On 07/07/16 08:32, Jan Beulich wrote:
>>>>>>> We must not skip the transaction_end() call for a failed
>>>>>>> XS_TRANSACTION_START. The removed code fragment got introduced by
>>>>>>> commit 027bd7e899 ("xen/xenbus: Avoid synchronous wait on XenBus
>>>>>>> stalling shutdown/restart") without its description really indicating
>>>>>>> why it was added (and hence I can't identify whether a more complex
>>>>>>> change might be needed here).
>>>>>>
>>>>>> If sending the XS_TRANSACTION_END message failed, then the transaction
>>>>>> is still open and transaction_end() should not be called.
>>>>>>
>>>>>> However, if sending an XS_TRANSACTION_START failed, then
>>>>>> transaction_end() should be called.
>>>>>>
>>>>>> So, yes a more complex fix is needed here.
>>>>>
>>>>> Well, both of the things you name are what happens with the patch
>>>>> in place. So if those two conditions are all that needs to be satisfied,
>>>>> then no more complex change is needed afaict (and was the behavior
>>>>> before the cross referenced commit) - the question really is whether
>>>>> that other commit meant to deal with something _beyond_ those two
>>>>> things.
>>>>
>>>> You call transaction_end() if msg->type == XS_TRANSACTION_END, even if
>>>> xb_write() returned an error.
>>>
>>> When xb_write() returns an error, msg->type gets set to XS_ERROR.
>> 
>> So?
>> 
>> 	if ((msg->type == XS_TRANSACTION_END) ||
>> 	    (...))
>> 		transaction_end();
>> 
>> We don't check msg->type for XS_TRANSACTION_END messages.
> 
> Sorry, being stupid.  Yeah, the change is fine but it needs a better
> commit message.

I can certainly omit the part in parentheses. I don't think I should
omit the reference to the original commit having introduced the issue.
And without a more specific hint I also don't know what else may
need changing. I'm sorry, I know I'm not doing very well in writing
commit messages to your liking.

Jan