linux-kernel - Re: lockdep trace when unplugging usb audio (.39rc4)

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Date:	Wed, 20 Apr 2011 12:43:27 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jaroslav Kysela <perex@...ex.cz>
Subject: Re: lockdep trace when unplugging usb audio (.39rc4)

At Wed, 20 Apr 2011 12:32:44 +0200,
Oliver Neukum wrote:
> 
> Am Dienstag, 19. April 2011, 20:07:45 schrieb Dave Jones:
> > I get this trace when I unplug my NuForce uDAC.
> 
> Please test this patch.

Well, no, it would break the real fix for disconnection race we've
done recently.

Such a lockdep warning is sometimes seen, but I haven't seen the real
deadlock by this in practice.  It's a path from munmap -> release, and
hits many other subsystems like NFS, if not fixed yet...


thanks,

Takashi

> 
> 	Regards
> 		Oliver
> [2 0001-usb-audio-fix-potential-deadlock-on-unplug.patch <text/x-patch; UTF-8 (quoted-printable)>]
> From 24d036ccb418d7cfa5a821d5a70b3d3da2d63bb3 Mon Sep 17 00:00:00 2001
> From: Oliver Neukum <oliver@...kum.org>
> Date: Wed, 20 Apr 2011 12:29:22 +0200
> Subject: [PATCH] usb-audio:fix potential deadlock on unplug
> 
> Decrease the scope of a lock to fix a deadlock
> 
> Signed-off-by: Oliver Neukum <oneukum@...e.de>
> ---
>  sound/usb/card.c |    3 +--
>  1 files changed, 1 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/usb/card.c b/sound/usb/card.c
> index a90662a..1d3d9f0 100644
> --- a/sound/usb/card.c
> +++ b/sound/usb/card.c
> @@ -554,6 +554,7 @@ static void snd_usb_audio_disconnect(struct usb_device *dev, void *ptr)
>  	mutex_lock(&chip->shutdown_mutex);
>  	chip->shutdown = 1;
>  	chip->num_interfaces--;
> +	mutex_unlock(&chip->shutdown_mutex);
>  	if (chip->num_interfaces <= 0) {
>  		snd_card_disconnect(card);
>  		/* release the pcm resources */
> @@ -569,11 +570,9 @@ static void snd_usb_audio_disconnect(struct usb_device *dev, void *ptr)
>  			snd_usb_mixer_disconnect(p);
>  		}
>  		usb_chip[chip->index] = NULL;
> -		mutex_unlock(&chip->shutdown_mutex);
>  		mutex_unlock(&register_mutex);
>  		snd_card_free_when_closed(card);
>  	} else {
> -		mutex_unlock(&chip->shutdown_mutex);
>  		mutex_unlock(&register_mutex);
>  	}
>  }
> -- 
> 1.7.1
> 
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