[PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[Dongli Zhang]

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);
-		}
 
 		/* The above kthread_stop() guarantees that at this point we
 		 * don't have any discard_io or other_io requests. So, checking
-- 
2.7.4

Re: [PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[Roger Pau Monné]

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>

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

kthread_stop waits for the thread to exit, so it must obviously wake
it up.

Thanks, Roger.

Re: [PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[Roger Pau Monné]

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?

Thanks, Roger.

Re: [Xen-devel] [PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[Dongli Zhang]

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

Re: [Xen-devel] [PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[Roger Pau Monné]

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.

Re: [Xen-devel] [PATCH 1/1] xen-blkback: do not wake up shutdown_wq after xen_blkif_schedule() is stopped

[Dongli Zhang]

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