netdev - Re: [PATCH RFC net-next] netvsc: get rid of completion timeouts

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Message-ID: <87inxjacnu.fsf@vitty.brq.redhat.com>
Date:	Wed, 08 Jun 2016 18:05:57 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	netdev@...r.kernel.org
Cc:	devel@...uxdriverproject.org, linux-kernel@...r.kernel.org,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: Re: [PATCH RFC net-next] netvsc: get rid of completion timeouts

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> I'm hitting 5 second timeout in rndis_filter_set_rss_param() while setting
> RSS parameters for the device. When this happens we end up returning
> -ETIMEDOUT from the function and rndis_filter_device_add() falls back to
> setting
>
> 	net_device->max_chn = 1;
>         net_device->num_chn = 1;
>         net_device->num_sc_offered = 0;
>
> but after a moment the rndis request succeeds and subchannels start to
> appear. netvsc_sc_open() does unconditional nvscdev->num_sc_offered-- and
> it becomes U32_MAX-1. Consequent rndis_filter_device_remove() will hang
> while waiting for all U32_MAX-1 subchannels to appear and this is not
> going to happen.
>
> The immediate issue could be solved by adding num_sc_offered > 0 check to
> netvsc_sc_open() but we're getting out of sync with the host and it's not
> easy to adjust things later, e.g. in this particular case we'll be creating
> queues without a user request for it and races are expected. Same applies
> to other parts of the driver which have the same completion timeout.
>
> Following the trend in drivers/hv/* code I suggest we remove all these
> timeouts completely. As a guest we can always trust the host we're running
> on and if the host screws things up there is no easy way to recover anyway.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>

Kbuild test robot reports an unused variable after the patch, I'll fix
this and resend together with a related fix so please don't apply this
RFC to net-next atm.

[skip]

-- 
  Vitaly