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

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Date:	Mon, 17 Nov 2008 15:18:28 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, David Miller <davem@...emloft.net>,
	Ferenc Wagner <wferi@...f.hu>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] softirq: Use local_irq_save() in local_bh_enable()

On Mon, 2008-11-17 at 13:35 +0000, Jarek Poplawski 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.

Unfortunately my brain lacks the magic to decrypt x86 stack traces, so
I'm unable to read much from that report other than that it hit the
WARN_ON. That looks more like the TX path to me? Anyway, my patch made
that trigger for everybody rather than just on NOPREEMPT/UP (or
something like that) and made the code easier to understand by removing
the flags that are pointless anyway if the API is used correctly.

You can find discussion around the patch at
http://lkml.org/lkml/2008/6/17/259

> 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.

Do we have evidence of this actually hitting often? This is the first
report of anything going wrong that I've seen ever since a single one
right after this commit went into testing five months ago.

IFF we want to add this back (and I'm not in favour) then please add a
big comment that this is only to accomodate broken users.

johannes

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