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Subject: Re: [Xen-devel] [PATCH 1/1] xen-blkback: do not wake up shutdown_wq
 after xen_blkif_schedule() is stopped
To:     =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>
References: <1547646461-29803-1-git-send-email-dongli.zhang@oracle.com>
 <20190116145215.igakzdkklc6f7h6k@mac>
 <30770f68-34e0-d707-b087-c43f0b51cd81@oracle.com>
 <20190117082043.7y4gl4uave5pgawf@mac>
Cc:     linux-block@vger.kernel.org, xen-devel@lists.xenproject.org,
        axboe@kernel.dk, linux-kernel@vger.kernel.org,
        konrad.wilk@oracle.com
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <00a7b217-ead4-06e2-2eaa-56c6d1920372@oracle.com>
Date:   Thu, 17 Jan 2019 18:23:54 +0800
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Hi Roger,

On 01/17/2019 04:20 PM, Roger Pau Monné wrote:
> On Thu, Jan 17, 2019 at 10:10:00AM +0800, Dongli Zhang wrote:
>> Hi Roger,
>>
>> On 2019/1/16 下午10:52, Roger Pau Monné wrote:
>>> On Wed, Jan 16, 2019 at 09:47:41PM +0800, Dongli Zhang wrote:
>>>> There is no need to wake up xen_blkif_schedule() as kthread_stop() is able
>>>> to already wake up the kernel thread.
>>>>
>>>> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
>>>> ---
>>>>  drivers/block/xen-blkback/xenbus.c | 4 +---
>>>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
>>>> index a4bc74e..37ac59e 100644
>>>> --- a/drivers/block/xen-blkback/xenbus.c
>>>> +++ b/drivers/block/xen-blkback/xenbus.c
>>>> @@ -254,10 +254,8 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
>>>>  		if (!ring->active)
>>>>  			continue;
>>>>  
>>>> -		if (ring->xenblkd) {
>>>> +		if (ring->xenblkd)
>>>>  			kthread_stop(ring->xenblkd);
>>>> -			wake_up(&ring->shutdown_wq);
>>>
>>> I've now realized that shutdown_wq is basically useless, and should be
>>> removed, could you please do it in this patch?
>>
>> I do not think shutdown_wq is useless.
>>
>> It is used to halt the xen_blkif_schedule() kthread when
>> RING_REQUEST_PROD_OVERFLOW() returns true in __do_block_io_op():
>>
>> 1121 static int
>> 1122 __do_block_io_op(struct xen_blkif_ring *ring)
>> ... ...
>> 1134         if (RING_REQUEST_PROD_OVERFLOW(&blk_rings->common, rp)) {
>> 1135                 rc = blk_rings->common.rsp_prod_pvt;
>> 1136                 pr_warn("Frontend provided bogus ring requests (%d - %d =
>> %d). Halting ring processing on dev=%04x\n",
>> 1137                         rp, rc, rp - rc, ring->blkif->vbd.pdevice);
>> 1138                 return -EACCES;
>> 1139         }
>>
>>
>> If there is bogus/invalid ring requests, __do_block_io_op() would return -EACCES
>> without modifying prod/cons index.
>>
>> Without shutdown_wq (just simply assuming we remove the below code without
>> handling -EACCES in xen_blkif_schedule()), the kernel thread would continue the
>> while loop.
>>
>> 648                 if (ret == -EACCES)
>> 649                         wait_event_interruptible(ring->shutdown_wq,
>> 650                                                  kthread_should_stop());
>>
>>
>> If xen_blkif_be_int() is triggered again (let's assume there is no optimization
>> on guest part and guest would send event for every request it puts on ring
>> buffer), we may come to do_block_io_op() again.
>>
>>
>> As the prod/cons index are not modified last time the code runs into
>> do_block_io_op() to process bogus request, we would hit the bogus request issue
>> again.
>>
>>
>> With shutdown_wq, the kernel kthread is blocked forever until such queue/ring is
>> destroyed. If we remove shutdown_wq, we are changing the policy to handle bogus
>> requests on ring buffer?
> 
> AFAICT the only wakeup call to shutdown_wq is removed in this patch,
> hence waiting on it seems useless. I would replace the
> wait_event_interruptible call in xen_blkif_schedule with a break, so
> that the kthread ends as soon as a bogus request is found. I think
> there's no point in waiting for xen_blkif_disconnect to stop the
> kthread.
> 
> Thanks, Roger.
> 

My fault. The shutdown_wq is useless. I think I was going to say
wait_event_interruptible() is useful as it is used to halt the kthread when
there is bogus request.

It is fine to replace wait_event_interruptible with a break to exit immediately
when there is bogus request.

Thank you very much!

Dongli Zhang