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

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Date:   Thu, 28 Mar 2019 19:00:18 +0000
From:   Haiyang Zhang <haiyangz@...rosoft.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        Haiyang Zhang <haiyangz@...uxonhyperv.com>
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 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);