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Date:	Wed, 18 May 2016 10:13:59 +0200
From:	Jesper Dangaard Brouer <>
To:	Jason Wang <>
Cc:, Eric Dumazet <>,,,,, Steven Rostedt <>
Subject: Re: [PATCH net-next] tuntap: introduce tx skb ring

On Mon, 16 May 2016 15:51:48 +0800
Jason Wang <> wrote:

> On 2016年05月16日 11:56, Eric Dumazet wrote:
> > On Mon, 2016-05-16 at 09:17 +0800, Jason Wang wrote:  
> >> We used to queue tx packets in sk_receive_queue, this is less
> >> efficient since it requires spinlocks to synchronize between producer
> >> and consumer.  
> > ...
> >  
> >>   	struct tun_struct *detached;
> >> +	/* reader lock */
> >> +	spinlock_t rlock;
> >> +	unsigned long tail;
> >> +	struct tun_desc tx_descs[TUN_RING_SIZE];
> >> +	/* writer lock */
> >> +	spinlock_t wlock;
> >> +	unsigned long head;
> >>   };
> >>     
> > Ok, we had these kind of ideas floating around for many other cases,
> > like qdisc, UDP or af_packet sockets...
> >
> > I believe we should have a common set of helpers, not hidden in
> > drivers/net/tun.c but in net/core/skb_ring.c or something, with more
> > flexibility (like the number of slots)
> >  
> Yes, this sounds good.

I agree. It is sad to see everybody is implementing the same thing,
open coding an array/circular based ring buffer.  This kind of code is
hard to maintain and get right with barriers etc.  We can achieve the
same performance with a generic implementation, by inlining the help
function calls.

I implemented an array based Lock-Free/cmpxchg based queue, that you
could be inspired by, see:

The main idea behind my implementation is bulking, to amortize the
locked cmpxchg operation. You might not need it now, but I expect we
need it in the future.

You cannot use my alf_queue directly as your "struct tun_desc" is
larger than one-pointer (which the alf_queue works with).  But it
should be possible to extend to handle larger "objects".

Maybe Steven Rostedt have an even better ring queue implementation
already avail in the kernel?

Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of

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