netdev - RE: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending refcount

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D458F61@AcuExch.aculab.com>
Date:	Mon, 13 Jan 2014 09:55:18 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Cong Wang' <cwang@...pensource.com>,
	Daniel Borkmann <dborkman@...hat.com>
CC:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending
 refcount

From: Cong Wang
> On Sun, Jan 12, 2014 at 8:22 AM, Daniel Borkmann <dborkman@...hat.com> wrote:
> > +static void packet_inc_pending(struct packet_ring_buffer *rb)
> > +{
> > +       this_cpu_inc(*rb->pending_refcnt);
> > +}
> > +
> > +static void packet_dec_pending(struct packet_ring_buffer *rb)
> > +{
> > +       this_cpu_dec(*rb->pending_refcnt);
> > +}
> > +
> > +static int packet_read_pending(const struct packet_ring_buffer *rb)
> > +{
> > +       int i, refcnt = 0;
> > +
> > +       /* We don't use pending refcount in rx_ring. */
> > +       if (rb->pending_refcnt == NULL)
> > +               return 0;
> > +
> > +       for_each_possible_cpu(i)
> > +               refcnt += *per_cpu_ptr(rb->pending_refcnt, i);
> > +
> > +       return refcnt;
> > +}
> 
> How is this supposed to work? Since there is no lock,
> you can't read accurate refcnt. Take a look at lib/percpu_counter.c.
> 
> I guess for some reason you don't care the accuracy?
> Then at least you need to comment in the code.

Hmmm... did the code ever work?

The value looks like the number of active transmits (s/pending/tx_pending/)
The test of the number of pending tx is at the bottom of a code loop,
I'd expect that some action should be locked against this check - but even
the atomic read doesn't do this.

Check what happens if the count reads as 1 just before another cpu
decrements it to zero (or reads 0 just before being incremented).
In one of those cases something is likely to go wrong.

I'm actually surprised that there isn't a mutex covering the code path.

	David



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