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@xxxxxxxxxx>
>> ---
>> 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
