netdev - Re: Q: what protects dev->napi

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Date:	Fri, 24 Aug 2012 12:12:22 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	netdev@...r.kernel.org,
	Sylvain Munaut <s.munaut@...tever-company.com>,
	David Miller <davem@...emloft.net>
Subject: Re: Q: what protects dev->napi_list?

On Fri, 2012-08-24 at 17:46 +0800, Cong Wang wrote:
> Hi,
> 
> Sylvain reported a netpoll CPU stall
> http://marc.info/?l=linux-netdev&m=134563282530588&w=2
> 
> I tried to provide some fix for it:
> http://marc.info/?l=linux-netdev&m=134571069921429&w=2
> 
> When reviewing that code, I noticed a problem, it seems dev->napi_list
> is not protected by any lock? What if the device driver calls
> netif_napi_del() meanwhile we are iterating &dev->napi_list in
> poll_napi()? It seems netif_napi_del()/netif_napi_add() are usually
> called with the RTNL lock held during driver init/uninit, but again
> poll_napi() doesn't have RTNL lock.
> 

Of course poll_napi() cant try to get RTNL (its a mutex by the way)

There are no problems, since :

netif_napi_add() is called at device open time (before napi_poll() can
use it)

netif_napi_del() at device dismantle time (after making sure napi_poll()
wont use the device again)



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