netdev - Re: [Question] Usage of dev_hold()/dev

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Date:	Tue, 25 Aug 2015 07:24:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Zhangjie (HZ)" <zhangjie14@...wei.com>
Cc:	Jason Wang <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Qinchuanyu <qinchuanyu@...wei.com>, Yewudi <yewudi@...wei.com>,
	liuyongan 00175866 <l00175866@...esmail.huawei.com.cn>,
	Wangbicheng <wangbicheng@...wei.com>,
	Yan Chen <Y.Chen@...wei.com>
Subject: Re: [Question] Usage of dev_hold()/dev_put()

On Tue, 2015-08-25 at 08:28 +0000, Zhangjie (HZ) wrote:
> Hi,
> 
> The comment of function dev_hold() and dev_put() are really simple.
> 
> Actually, I can’t find a rule to follow.
> 
> Where should I call dev_hold()/dev_put()?
> 
> Are they necessary during xmit or receive or only in register/release?
> 
> I find that, for tap, it calls dev_hold()/dev_put() for every
> tun_sendmsg/tun_recvmsg:
> 
> So, should I call dev_hold()/dev_put() for each quote, such like:
> “skb->dev = dev” ?
> 
> But, for physical nic, I can’t find dev_hold()/dev_put() during xmit
> or receive.
> 
> What kind of scene is it necessary to call dev_hold()/dev_put()?
> 
>  
> 
> Look forward for your feedback. 
> 
> Thank you ! J
> 
> Zhangjie
> 
> 
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In general, you need to use dev_hold() for every reference on 'dev'
stored in the object.

However they are some paths were you do not need that : For example in
transmit path, skb are stored either in a qdisc or device driver
internal queue/ring. At device dismantle we properly delete all these
skb, so we do not have to worry about used dev_hold()/dev_put() in
transmit.

In drivers/net/tun.c, the dev_hold()/dev_put() only is required so that
device doesnt disappear between tun_get() and actual use of it after a
potentially long copy from user space (that might trigger page faults)



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