netdev - RE: [PATCH hyperv-fixes] hv_netvsc: Fix unwanted wakeup after tx

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Message-ID: <DM6PR21MB13373F19D06D1FB27B65A9F6CA590@DM6PR21MB1337.namprd21.prod.outlook.com>
Date:   Thu, 28 Mar 2019 19:59:05 +0000
From:   Haiyang Zhang <haiyangz@...rosoft.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
CC:     "sashal@...nel.org" <sashal@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "olaf@...fle.de" <olaf@...fle.de>, vkuznets <vkuznets@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH hyperv-fixes] hv_netvsc: Fix unwanted wakeup after
 tx_disable



> -----Original Message-----
> From: Stephen Hemminger <stephen@...workplumber.org>
> Sent: Thursday, March 28, 2019 3:42 PM
> To: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...uxonhyperv.com>; sashal@...nel.org;
> linux-hyperv@...r.kernel.org; KY Srinivasan <kys@...rosoft.com>; Stephen
> Hemminger <sthemmin@...rosoft.com>; olaf@...fle.de; vkuznets
> <vkuznets@...hat.com>; davem@...emloft.net; netdev@...r.kernel.org;
> linux-kernel@...r.kernel.org
> Subject: Re: [PATCH hyperv-fixes] hv_netvsc: Fix unwanted wakeup after
> tx_disable
> 
> On Thu, 28 Mar 2019 19:00:18 +0000
> Haiyang Zhang <haiyangz@...rosoft.com> wrote:
> 
> > > -----Original Message-----
> > > From: Stephen Hemminger <stephen@...workplumber.org>
> > > Sent: Thursday, March 28, 2019 2:38 PM
> > > To: Haiyang Zhang <haiyangz@...uxonhyperv.com>
> > > Cc: sashal@...nel.org; linux-hyperv@...r.kernel.org; Haiyang Zhang
> > > <haiyangz@...rosoft.com>; KY Srinivasan <kys@...rosoft.com>;
> Stephen
> > > Hemminger <sthemmin@...rosoft.com>; olaf@...fle.de; vkuznets
> > > <vkuznets@...hat.com>; davem@...emloft.net;
> netdev@...r.kernel.org;
> > > linux-kernel@...r.kernel.org
> > > Subject: Re: [PATCH hyperv-fixes] hv_netvsc: Fix unwanted wakeup
> > > after tx_disable
> > >
> > > On Thu, 28 Mar 2019 17:48:45 +0000
> > > Haiyang Zhang <haiyangz@...uxonhyperv.com> wrote:
> > >
> > > > +static inline void netvsc_tx_enable(struct netvsc_device *nvscdev,
> > > > +				    struct net_device *ndev)
> > > > +{
> > > > +	nvscdev->tx_disable = false;
> > > > +	mb(); /* ensure queue wake up mechanism is on */
> > > > +
> > > > +	netif_tx_wake_all_queues(ndev);
> > > > +}
> > >
> > > You don't need a full mb(). virt_wmb() should be sufficient.
> >
> > I will make this change.
> >
> > > Could I suggest an alternative approach.
> > > You don't need to introduce a local tx_disable flag, the only place
> > > where a wakeup could cause problems is after a send_completion was
> > > processed during detach state.
> > >
> > > Instead, just avoid wakeup in that place.
> >
> > In netvsc_detach(), after netif_tx_disable(), we call
> > netvsc_wait_until_empty(nvdev); TX patch should not be waken up again
> while waiting for in/out ring to becomes empty.
> >
> > In my tests before this patch, there are wakeup happens before
> > netif_device_detach(), so netif_device_present(ndev) is still true at that
> time.
> >
> > In other places, like netvsc_close(), link_change(), we also don't want
> wakeup after tx_disable.
> >
> > Thanks.
> > - Haiyang
> >
> > >
> > > --- a/drivers/net/hyperv/netvsc.c
> > > +++ b/drivers/net/hyperv/netvsc.c
> > > @@ -720,6 +720,7 @@ static void netvsc_send_tx_complete(struct
> > > net_device *ndev,
> > >                 struct netdev_queue *txq = netdev_get_tx_queue(ndev,
> > > q_idx);
> > >
> > >                 if (netif_tx_queue_stopped(txq) &&
> > > +                   netif_device_present(ndev) &&
> > >                     (hv_get_avail_to_write_percent(&channel->outbound) >
> > >                      RING_AVAIL_PERCENT_HIWATER || queue_sends < 1)) {
> > >                         netif_tx_wake_queue(txq);
> 
> 
> Then what about doing netif_detach earlier in netvsc_detach.
> 
> The state management is already (too) complex in netvsc and adding another
> boolean flag just makes it harder to understand.

If we move netif_device_detach() before the netvsc_wait_until_empty(), the remaining
Packets in the receive buffer will be passed to a detached device.

Also, in case of netvsc_close() and link_change(), we don't call netif_device_detach().

Thanks,
- Haiyang