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

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Message-ID: <20081118074922.GB4440@ff.dom.local>
Date:	Tue, 18 Nov 2008 07:49:22 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
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, Nov 17, 2008 at 03:18:28PM +0100, Johannes Berg wrote:
> 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?

OK, this looks like both paths (which is probably common in networking).

> 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

Yes, it's very interesting.

> 
> > 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.

Yes, it seems there should be more such reports from netconsole users.
But, I guess we kind of expect this if we still use WARN_ON and not
BUG_ON here?

Jarek P.
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