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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>
Cc:     linux-block@vger.kernel.org, xen-devel@lists.xenproject.org,
        axboe@kernel.dk, linux-kernel@vger.kernel.org,
        konrad.wilk@oracle.com
References: <1547646461-29803-1-git-send-email-dongli.zhang@oracle.com>
 <20190116145215.igakzdkklc6f7h6k@mac>
From:   Dongli Zhang <dongli.zhang@oracle.com>
Message-ID: <30770f68-34e0-d707-b087-c43f0b51cd81@oracle.com>
Date:   Thu, 17 Jan 2019 10:10:00 +0800
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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?

Please correct me if my understanding is wrong.

Thank you very much!

Dongli Zhang