netdev - Re: question about napi_disable (was Re: [PATCH] virtio_net: set/cancel work on ndo_open/ndo

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Date:	Thu, 05 Apr 2012 14:32:03 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Amit Shah <amit.shah@...hat.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: question about napi_disable (was Re: [PATCH] virtio_net: set/cancel
 work on ndo_open/ndo_stop)

On 04/04/2012 05:32 PM, Michael S. Tsirkin wrote:
> On Thu, Dec 29, 2011 at 09:12:38PM +1030, Rusty Russell wrote:
>> >  Michael S. Tsirkin noticed that we could run the refill work after
>> >  ndo_close, which can re-enable napi - we don't disable it until
>> >  virtnet_remove.  This is clearly wrong, so move the workqueue control
>> >  to ndo_open and ndo_stop (aka. virtnet_open and virtnet_close).
>> >  
>> >  One subtle point: virtnet_probe() could simply fail if it couldn't
>> >  allocate a receive buffer, but that's less polite in virtnet_open() so
>> >  we schedule a refill as we do in the normal receive path if we run out
>> >  of memory.
>> >  
>> >  Signed-off-by: Rusty Russell<rusty@...tcorp.com.au>
> Doh.
> napi_disable does not prevent the following
> napi_schedule, does it?
>
> Can someone confirm that I am not seeing things please?
Looks like napi_disable() does prevent the following scheduling, as 
napi_schedule_prep() returns true only when there's an 0 -> 1 transition 
of NAPI_STATE_SCHED bit.
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