netdev - Re: [PATCH] softirq: Use local_irq_save() in local_bh

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Message-ID: <20081118073819.GA4440@ff.dom.local>
Date:	Tue, 18 Nov 2008 07:38:19 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Johannes Berg <johannes@...solutions.net>,
	David Miller <davem@...emloft.net>,
	Ferenc Wagner <wferi@...f.hu>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] softirq: Use local_irq_save() in local_bh_enable()

On Mon, Nov 17, 2008 at 05:16:17PM +0100, Ingo Molnar wrote:
> 
> * Jarek Poplawski <jarkao2@...il.com> wrote:
> 
> > This report: http://marc.info/?l=linux-netdev&m=122599341430090&w=2 
> > shows local_bh_enable() is used in the wrong context (irqs 
> > disabled). It happens when a usual network receive path is called by 
> > netconsole, which simply turns off irqs around this all. Probably 
> > this is wrong, but it worked like this long time, and it's not 
> > trivial to fix this.
> > 
> > Anyway, a commit 0f476b6d91a1395bda6464e653ce66ea9bea7167 "softirq: 
> > remove irqs_disabled warning from local_bh_enable" can break things 
> > after changing from local_irq_save() to local_irq_disable(). Before 
> > this commit there was only a warning, now a lockup is possible, so 
> > it could be treated as a regression. This patch reverts the change 
> > in irqs.
> 
> hm, but calling local_bh_enable() with hardirqs off is a bug. It might 
> be a long-standing bug, but it can cause lockups even with that change 
> reverted: when we process softirqs in local_bh_enable().

I think it's what they call a regression: this is a long-standing bug,
and this commit doesn't fix this, but can cause additional lockups.

> So why not 
> fix the bug instead?

It's not about instead: this bug could be fixed as well (if somebody
knows how to do it "properly" without hacks like:
	if (!in_irq())
		local_bh_disable();
etc.; but, I guess, the network code has more such bh disabling).

Jarek P.
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