Date: Thu, 17 Feb 2005 08:52:12 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "David S. Miller" <davem@davemloft.net>
Cc: mgross@linux.intel.com, rostedt@goodmis.org, linux-kernel@vger.kernel.org,
       Mark_H_Johnson@raytheon.com
Subject: Re: queue_work from interrupt Real time preemption2.6.11-rc2-RT-V0.7.37-03
Message-ID: <20050217075212.GA21621@elte.hu>
References: <200502141240.14355.mgross@linux.intel.com> <200502141429.11587.mgross@linux.intel.com> <20050215104153.GB19866@elte.hu> <200502151006.44809.mgross@linux.intel.com> <20050216051645.GB15197@elte.hu> <20050216081143.50d0a9d6.davem@davemloft.net>
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* David S. Miller <davem@davemloft.net> wrote:

> > Maybe the networking
> > stack would break if we allowed the TIMER softirq (thread) to preempt
> > the NET softirq (threads) (and vice versa)?
> 
> The major assumption is that softirq's run indivisibly per-cpu.
> Otherwise the per-cpu queues of RX and TX packet work would get
> corrupted.

as long as it stays on a single CPU, could we allow softirq contexts to
preempt each other? I.e. we'd keep the per-CPU assumption (that is fair
and needed for performance anyway), but we'd allow NET_TX to preempt
NET_RX and vice versa. Would this corrupt the rx/tx queues? (i suspect
it would.)

(anyway, by adding an explicit no-preempt section around the 'take
current rx queue private, then process it' on PREEMPT_RT it could be
made safe. I'm wondering whether there are any other deeper assumptions
about atomic separation of softirq contexts.)

	Ingo
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