netdev - Re: RFH, where did I go wrong?

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Message-ID: <93ce7034-a26f-b68d-f27f-ef90b6b01bf8@tarent.de>
Date:   Wed, 12 Oct 2022 23:56:11 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     Andrew Lunn <andrew@...n.ch>
cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Dave Taht <dave.taht@...il.com>, netdev@...r.kernel.org
Subject: Re: RFH, where did I go wrong?

On Wed, 12 Oct 2022, Andrew Lunn wrote:

> > Ooh! Will try! That’s what I get for getting, ahem, inspiration
> > from other qdiscs.
> 
> Are other qdiscs also missing RTNL, or are you just using the
> inspiration in a different context?

I think I was probably confused between which of the functions can
be used when. Eric explained the why. What I was missing was… well,
basically what I asked for weeks ago — what functions I need to
provide when writing a qdisc, and which guarantees and expectations
these have. That rtnl is held for… apparently all but enqueue/dequeue…
was one of these. I doubt other qdiscs miss it, or their users would
also run into this crash or so :/

The thing I did first was to add ASSERT_RTNL(); directly before the
rtnl_* call, just like it was in the other place. That, of course,
crashed immediately. Now *this* could be done systematically.

In OpenBSD, things like that are often hidden behind #if DIAGNOSTIC
which is a global option, disabled in “prod” or space-constrained
(installer) kernels but enabled for the “generic” one for wide testing.
Something to think about?

I’m sure there’s lots of things like flow analysis around in the
Linux world, however that wouldn’t help out-of-tree code being
developed, whereas extra checks like that would. Just some thoughts,
as said earlier this is basically¹ my start in Linux kernel dev.

bye,
//mirabilos
① some small patching of existing code over the years excepted
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