netdev - Q: what protects dev->napi

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Message-ID: <1345801604.11584.24.camel@cr0>
Date:	Fri, 24 Aug 2012 17:46:44 +0800
From:	Cong Wang <amwang@...hat.com>
To:	netdev@...r.kernel.org
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Sylvain Munaut <s.munaut@...tever-company.com>,
	David Miller <davem@...emloft.net>
Subject: Q: what protects dev->napi_list?

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.

Am I missing anything?

Thanks!

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