linux-kernel - Re: [Xen-devel] [PATCH] xen/scsifront: don't advance ring request pointer in case of error

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Date:   Tue, 29 Nov 2016 13:33:02 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     lambert.quentin@...il.com, jejb@...ux.vnet.ibm.com,
        xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
        dan.carpenter@...cle.com, martin.petersen@...cle.com,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen/scsifront: don't advance ring request
 pointer in case of error

On 29/11/16 12:40, Jan Beulich wrote:
>>>> On 29.11.16 at 12:19, <JGross@...e.com> wrote:
>> On 29/11/16 12:14, Jan Beulich wrote:
>>>>>> On 29.11.16 at 11:50, <JGross@...e.com> wrote:
>>>> --- a/drivers/scsi/xen-scsifront.c
>>>> +++ b/drivers/scsi/xen-scsifront.c
>>>> @@ -184,8 +184,6 @@ static struct vscsiif_request *scsifront_pre_req(struct 
>> vscsifrnt_info *info)
>>>>  
>>>>  	ring_req = RING_GET_REQUEST(&(info->ring), ring->req_prod_pvt);
>>>>  
>>>> -	ring->req_prod_pvt++;
>>>
>>> Please note the "_pvt" suffix, which stands for "private": This field is
>>> not visible to the backend. Only ring->sring fields are shared, and
>>> the updating of the shared field happens in RING_PUSH_REQUESTS()
>>> and RING_PUSH_REQUESTS_AND_CHECK_NOTIFY().
>>
>> Sure, but RING_PUSH_REQUESTS() will copy req_prod_pvt to req_prod. In
>> the case corrected this would advance req_prod by two after the error
>> case before, even if only one request would have made it to the ring.
> 
> Okay, then I may have been mislead by the patch description: I
> understood it to say that you want to avoid the backend seeing
> requests which haven't been filled fully, but it looks like you're
> instead saying that for these requests the filling will never be
> completed (because of some unrelated(?) error). Iirc other
> frontend drivers behave similarly to the unpatched scsifront, and

blkfront and netfront seem to be okay.

> incrementing req_prod_pvt late has possible (perhaps just
> theoretical) other issues, like parallel retrieval and filling of them
> on mor than one CPU. Wouldn't it be better to obtain a request

In scsifront the complete critical path is guarded by a lock.

> structure only when everything else is ready (and hence no further
> errors can occur)? After all you also need to deal with the acquired
> ID upon errors, and seems odd to me to deal with the two parts of
> cleanup in different places (and even in different ways).

Hmm, I can see your point.

I'll have a look how intrusive such a change would be.


Juergen