netdev - Re: freeze with interface rename & SIOCGIFNAME

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Message-ID: <1372179447.3301.105.camel@edumazet-glaptop>
Date:	Tue, 25 Jun 2013 09:57:27 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nicolas Schichan <nschichan@...ebox.fr>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Cong Wang <amwang@...hat.com>
Subject: Re: freeze with interface rename & SIOCGIFNAME

On Tue, 2013-06-25 at 18:27 +0200, Nicolas Schichan wrote:
> On 06/25/2013 06:15 PM, Eric Dumazet wrote:
> > Nice catch !
> >
> > Please add a helper so that we can use it as well from
> > sock_getbindtodevice(), and submit an official patch ?
> 
> Sure, I'll do that.
> 
> > int netdev_get_name(char *name, int ifindex)
> > {
> > 	struct net_device *dev;
> > 	unsigned int seq;
> >
> > retry:
> >          seq = read_seqcount_begin(&devnet_rename_seq);
> 
> raw_seqcount_begin is required here here. read_seqcount_begin will spin 
> forever as the s->sequence & 1 check will always be true in 
> __read_seqcount_begin(). This will incur some unneed work if a rename is in 
> progress, but that's better than locking the system up.

I am not sure it's needed, because the writer holds a spinlock and is
not allowed to sleep or being preempted.

If it's needed, there is a bug somewhere else...



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