linux-kernel - Re: [PATCH] xen: xen-pciback: Remove create

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Message-ID: <574D636D.9010104@citrix.com>
Date:	Tue, 31 May 2016 11:11:57 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Tejun Heo <tj@...nel.org>
CC:	Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
	<boris.ostrovsky@...cle.com>, <jgross@...e.com>,
	<JBeulich@...e.com>, <paul.gortmaker@...driver.com>,
	<stefano.stabellini@...citrix.com>, <cardoe@...doe.com>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen: xen-pciback: Remove create_workqueue

On 27/05/16 17:32, Konrad Rzeszutek Wilk wrote:
> On Fri, May 27, 2016 at 12:08:22PM -0400, Tejun Heo wrote:
>> Hello,
>>
>> On Fri, May 27, 2016 at 12:01:14PM -0400, Konrad Rzeszutek Wilk wrote:
>>> On Fri, May 27, 2016 at 09:24:11PM +0530, Bhaktipriya Shridhar wrote:
>>>> With concurrency managed workqueues, use of dedicated workqueues can be
>>>> replaced by using system_wq. Drop host->intr_wq by using
>>                                       ^
>> 				      xen_pcibk_wq
>>>> system_wq.
>>>>
>>>> Since there is only a single work item, increase of concurrency level by
>>>> switching to system_wq should not break anything.
>>>
>>> _should_ not? Hehe. I presume this has not been tested?
>>
>> Yeah, this is a part of sweeping conversions and it's challenging (and
>> often impossible for specific drivers) to setup test environments.
>> xen isn't as bad but can still be a pretty specialized setup.  The
>> conversions aren't high risk and shouldn't be too difficult to root
>> cause when something goes south.  We'd greatly appreciate any helps
>> with reviewing and testing.
>>
>>>> cancel_work_sync() has been used in xen_pcibk_disconnect() to ensure that
>>>> work item is not pending or executing by the time exit path runs.
>>>>
>>>> Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@...il.com>
>>>> @@ -76,8 +75,7 @@ static void xen_pcibk_disconnect(struct xen_pcibk_device *pdev)
>>>>  	/* If the driver domain started an op, make sure we complete it
>>>>  	 * before releasing the shared memory */
>>>>
>>>> -	/* Note, the workqueue does not use spinlocks at all.*/
>>>> -	flush_workqueue(xen_pcibk_wq);
>>>> +	cancel_work_sync(&pdev->op_work);
>>
>> Should it be flush_work() instead?  Is it okay for a pdev->op_work to
>> be queued and canceled without actually getting executed?
> 
> It should really flush them. The comment above says so, but in reality it
> does not matter that much as we tearing down the communication. As long as
> the pdev->op_work either completes or is never executed - we are fine.

The comment should be updated to reflect this.

David