linux-kernel - Re: [PATCH] net: Adjust softirq raising in __napi

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Date:	Tue, 27 Oct 2009 08:01:40 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Tilman Schmidt <tilman@...p.cc>
Cc:	Jarek Poplawski <jarkao2@...il.com>,
	David Miller <davem@...emloft.net>, hidave.darkstar@...il.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	linux-wireless@...r.kernel.org, linux-ppp@...r.kernel.org,
	netdev@...r.kernel.org, paulus@...ba.org,
	Michael Buesch <mb@...sch.de>,
	Oliver Hartkopp <oliver@...tkopp.net>
Subject: Re: [PATCH] net: Adjust softirq raising in __napi_schedule

On Tue, 2009-10-27 at 01:52 +0100, Tilman Schmidt wrote:

> > Any code (say ISDN code) that calls netif_rx() is clearly assuming to
> > always be running in (soft)irq context, otherwise it couldn't call
> > netif_rx() unconditionally. Agree so far?
> 
> Well, in fact I'm not sure. :-) All I know is that in the ISDN case, no
> such assumption is explicitly stated anywhere. (The code in question is
> called from the rcvcallb_skb() callback method which the hardware driver
> calls when data has been received, and the description of that method in
> Documentation/isdn/INTERFACE does not say anything about the context in
> which it may be called.) The relevant code in drivers/isdn/i4l/isdn_ppp.c
> is rather old, perhaps even older than softirqs and the netif_rx() /
> netif_rx_ni() split. (Bear in mind that we are talking about the old
> ISDN4Linux subsystem which initially didn't even make it into the 2.6
> series because it was considered obsolete.) It seems quite possible to me
> that just no one ever thought about that question.

Heh :)

> > So now if you change the ISDN code to call netif_rx_ni(), you've changed
> > the assumption that the ISDN code makes -- that it is running in
> > (soft)irq context. Therefore, you need to verify that this is actually a
> > correct change, which is what I tried to say.
> 
> Understood. However, the fact that the local_softirq_pending message is
> appearing would seem to indicate that this assumption was wrong to
> begin with, wouldn't it?

I thought it only recently started appearing with a new driver or
something, but I may have misunderstood you. Anyway, I think that sums
up the issue from my POV.

johannes

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