Subject: Re: [PATCH net-next 1/1] hv_netvsc: Wait for sub-channels to be processed during probe

From: Vitaly Kuznetsov <[email-masked]>
Date: Fri, 17 Jul 2015 18:09:44 +0200

KY Srinivasan <[email-masked]> writes:

>> -----Original Message-----
>> From: Vitaly Kuznetsov [mailto:[email-masked]]
>> Sent: Friday, July 17, 2015 7:13 AM
>> To: KY Srinivasan
>> Cc: [email-masked]; [email-masked]; linux-
>> [email-masked]; [email-masked]; [email-masked];
>> [email-masked]; [email-masked]; Dexuan Cui
>> Subject: Re: [PATCH net-next 1/1] hv_netvsc: Wait for sub-channels to be
>> processed during probe
>> 
>> "K. Y. Srinivasan" <[email-masked]> writes:
>> 
>> > The current code returns from probe without waiting for the proper
>> handling
>> > of subchannels that may be requested. If the netvsc driver were to be
>> rapidly
>> > loaded/unloaded, we can  trigger a panic as the unload will be tearing
>> > down state that may not have been fully setup yet. We fix this issue by
>> making
>> > sure that we return from the probe call only after ensuring that the
>> > sub-channel offers in flight are properly handled.
>> >
>> > Signed-off-by: K. Y. Srinivasan <[email-masked]>
>> > Reviewed-and-tested-by: Haiyang Zhang <[email-masked]
>> > ---
>> >  drivers/net/hyperv/hyperv_net.h   |    2 ++
>> >  drivers/net/hyperv/rndis_filter.c |   25 +++++++++++++++++++++++++
>> >  2 files changed, 27 insertions(+), 0 deletions(-)
>> >
>> > diff --git a/drivers/net/hyperv/hyperv_net.h
>> b/drivers/net/hyperv/hyperv_net.h
>> > index 26cd14c..925b75d 100644
>> > --- a/drivers/net/hyperv/hyperv_net.h
>> > +++ b/drivers/net/hyperv/hyperv_net.h
>> > @@ -671,6 +671,8 @@ struct netvsc_device {
>> >  	u32 send_table[VRSS_SEND_TAB_SIZE];
>> >  	u32 max_chn;
>> >  	u32 num_chn;
>> > +	spinlock_t sc_lock; /* Protects num_sc_offered variable */
>> > +	u32 num_sc_offered;
>> >  	atomic_t queue_sends[NR_CPUS];
>> >
>> >  	/* Holds rndis device info */
>> > diff --git a/drivers/net/hyperv/rndis_filter.c
>> b/drivers/net/hyperv/rndis_filter.c
>> > index 2e40417..2e09f3f 100644
>> > --- a/drivers/net/hyperv/rndis_filter.c
>> > +++ b/drivers/net/hyperv/rndis_filter.c
>> > @@ -984,9 +984,16 @@ static void netvsc_sc_open(struct vmbus_channel
>> *new_sc)
>> >  	struct netvsc_device *nvscdev;
>> >  	u16 chn_index = new_sc->offermsg.offer.sub_channel_index;
>> >  	int ret;
>> > +	unsigned long flags;
>> >
>> >  	nvscdev = hv_get_drvdata(new_sc->primary_channel->device_obj);
>> >
>> > +	spin_lock_irqsave(&nvscdev->sc_lock, flags);
>> > +	nvscdev->num_sc_offered--;
>> > +	spin_unlock_irqrestore(&nvscdev->sc_lock, flags);
>> > +	if (nvscdev->num_sc_offered == 0)
>> > +		complete(&nvscdev->channel_init_wait);
>> > +
>> >  	if (chn_index >= nvscdev->num_chn)
>> >  		return;
>> >
>> > @@ -1015,8 +1022,10 @@ int rndis_filter_device_add(struct hv_device
>> *dev,
>> >  	u32 rsscap_size = sizeof(struct ndis_recv_scale_cap);
>> >  	u32 mtu, size;
>> >  	u32 num_rss_qs;
>> > +	u32 sc_delta;
>> >  	const struct cpumask *node_cpu_mask;
>> >  	u32 num_possible_rss_qs;
>> > +	unsigned long flags;
>> >
>> >  	rndis_device = get_rndis_device();
>> >  	if (!rndis_device)
>> > @@ -1039,6 +1048,8 @@ int rndis_filter_device_add(struct hv_device
>> *dev,
>> >  	net_device->max_chn = 1;
>> >  	net_device->num_chn = 1;
>> >
>> > +	spin_lock_init(&net_device->sc_lock);
>> > +
>> >  	net_device->extension = rndis_device;
>> >  	rndis_device->net_dev = net_device;
>> >
>> > @@ -1116,6 +1127,9 @@ int rndis_filter_device_add(struct hv_device
>> *dev,
>> >  	num_possible_rss_qs = cpumask_weight(node_cpu_mask);
>> >  	net_device->num_chn = min(num_possible_rss_qs, num_rss_qs);
>> >
>> > +	num_rss_qs = net_device->num_chn - 1;
>> > +	net_device->num_sc_offered = num_rss_qs;
>> > +
>> >  	if (net_device->num_chn == 1)
>> >  		goto out;
>> >
>> > @@ -1157,11 +1171,22 @@ int rndis_filter_device_add(struct hv_device
>> *dev,
>> >
>> >  	ret = rndis_filter_set_rss_param(rndis_device, net_device-
>> >num_chn);
>> >
>> > +	/*
>> > +	 * Wait for the host to send us the sub-channel offers.
>> > +	 */
>> > +	spin_lock_irqsave(&net_device->sc_lock, flags);
>> > +	sc_delta = net_device->num_chn - 1 - num_rss_qs;
>> > +	net_device->num_sc_offered -= sc_delta;
>> > +	spin_unlock_irqrestore(&net_device->sc_lock, flags);
>> > +
>> > +	if (net_device->num_sc_offered != 0)
>> > +		wait_for_completion(&net_device->channel_init_wait);
>> 
>> I'd suggest we add an essentian timeout (big, let's say 30 sec.)
>> here. In case something goes wrong we don't really want to hang the
>> whole kernel for forever. Such bugs are hard to debug as if a 'kernel
>> hangs' is reported we can't be sure which wait caused it. We can even
>> have something like:
>> 
>>  t = wait_for_completion_timeout(&net_device->channel_init_wait, 30*HZ);
>>  BUG_ON(t == 0);
>> 
>> This is much better as we'll be sure what went wrong. (I know other
>> pieces of hyper-v code use wait_for_completion() without a timeout, this
>> is rather a general suggestion for all of them).
>
> There is some history here. Initially, I had timeout for calls where we could reasonably
> rollback state if we timed out. Some calls were subsequently changed to unconditional
> wait because under some load conditions, these timeouts would trigger (granted I did not
> have 30 second timeout; it was a 5 second timeout).
>
> Greg was opposed to calls to BUG_ON() in general for drivers.

Even WARN() in my opinion should do the job: we'll at least have
something in the log.

> Consequently, I have chosen to
> wait unconditionally in cases where there is no sensible rollback if we were to timeout (rather
> thank bug checking).

(irrelevant to this patch but) in that case I'd suggest we have
something like:

while (1) {
     t = wait_for_completion_timeout(&..., 10*HZ);
     if (t)
         break;
     WARN(1, "Timeout while waiting for hypervisor to reply! Keep waiting...");
}

The sole purpose of that is to have something in dmesg.

>
> In this specific case though, we can have a timeout and we don't need to bug check if we timeout.
>
> I will make this change and resubmit.

Thanks!

>
> Thanks,
>
> K. Y 
>> 
>> >  out:
>> >  	if (ret) {
>> >  		net_device->max_chn = 1;
>> >  		net_device->num_chn = 1;
>> >  	}
>> > +
>> >  	return 0; /* return 0 because primary channel can be used alone */
>> >
>> >  err_dev_remv:
>> 
>> --
>>   Vitaly

-- 
  Vitaly