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

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