netdev - Re: WARNING: at net/ipv4/tcp_input.c:2927 tcp

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Message-ID: <20090328075628.3565eb39@bike.lwn.net>
Date:	Sat, 28 Mar 2009 07:56:28 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: WARNING: at net/ipv4/tcp_input.c:2927 tcp_ack+0xd55/0x1991()

On Sat, 28 Mar 2009 10:55:14 +0100
Markus Trippelsdorf <markus@...ppelsdorf.de> wrote:

> Yes, you might be right, because running with CONFIG_PROVE_LOCKING and 
> CONFIG_DETECT_SOFTLOCKUP enabled points to a possible bug in the BKL
> removal patches (fasync) by Jonathan Corbet. (I wasn't able so far to
> reproduce the original WARNING.)
> 
> Here is one example:
> 
> =========================================================
> [ INFO: possible irq lock inversion dependency detected ]
> 2.6.29-03321-gbe0ea69 #7
> ---------------------------------------------------------
> swapper/0 just changed the state of lock:
>  (fasync_lock){..+.}, at: [<ffffffff8028a2ac>] kill_fasync+0x24/0x45
> but this lock took another, hard-irq-unsafe lock in the past:
>  (&f->f_lock){--..}

That's not a bug; f_lock will never be taken in IRQ mode.  There's a
fix for the warning in linux-next now; my plan is to get it upstream
before -rc1.

Thanks,

jon

Jonathan Corbet / LWN.net / corbet@....net
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