netdev - Re: [STRAW MAN PATCH] sch_teql doesn't load-balance ppp(oatm) slaves

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Message-ID: <1332880522.2058.67.camel@shinybook.infradead.org>
Date:	Tue, 27 Mar 2012 21:35:22 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, paulus@...ba.org,
	netdev@...r.kernel.org
Subject: Re: [STRAW MAN PATCH] sch_teql doesn't load-balance ppp(oatm) slaves

On Tue, 2012-03-27 at 21:55 +0200, Eric Dumazet wrote:
> I have no idea why you added all these barriers... 

Um, some of them snuck in while I was working it out, and then I
couldn't prove to myself that they *weren't* needed, so I left them.

We definitely need a write barrier in pppoatm_pop() after the change to
sk->sk_sndbuf (which happens in the old_pop routine) and the increment
of pvcc->inflight. Those changes must hit *before* the test/change of
the BLOCKED bit.

But there's an implicit barrier in test_and_clear_bit() which should
achieve that, so the specific barrier you highlight may well be
superfluous. I could have sworn I had a reason for it at the time, but
can't justify it now.

On the pppoatm_may_send() side, the change to the BLOCKED bit needs a
corresponding read barrier after it, to ensure that its subsequent
checks of sk->sndbuf and pvcc->inflight are looking at the data which
were written before the BLOCKED bit is tested in pppoatm_pop().

But I suppose we can probably dispense with the barrier *before* setting
the BLOCKED bit in pppoatm_may_send(), and the barriers after increasing
pvcc->inflight.

If it looks sane other than that, I can knock up a new patch with a
S-O-B. Thanks for reviewing...

-- 
dwmw2

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