linux-kernel - Re: [bug, 2.6.26-rc4/rc5] sporadic bootup crashes in blk_lookup_devt()/prepare

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Message-ID: <20080609165757.184724ff@gondolin.boeblingen.de.ibm.com>
Date:	Mon, 9 Jun 2008 16:57:57 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Vegard Nossum" <vegard.nossum@...il.com>
Cc:	"Adrian Bunk" <bunk@...nel.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"Jens Axboe" <jens.axboe@...cle.com>,
	"Greg Kroah-Hartman" <gregkh@...e.de>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Kay Sievers" <kay.sievers@...y.org>, "Neil Brown" <neilb@...e.de>,
	"Mariusz Kozlowski" <m.kozlowski@...land.pl>,
	"Dave Young" <hidave.darkstar@...il.com>
Subject: Re: [bug, 2.6.26-rc4/rc5] sporadic bootup crashes in
 blk_lookup_devt()/prepare_namespace()

On Mon, 9 Jun 2008 16:28:09 +0200,
"Vegard Nossum" <vegard.nossum@...il.com> wrote:

> On 6/9/08, Vegard Nossum <vegard.nossum@...il.com> wrote:
> >  It seems that this list (block_class.devices) is protected by
> >  block_class_lock in block/genhd.c. This list is only ever modified by
> >  device_add() and device_del() in drivers/base/core.c. Both of those
> >  are (only) protected by dev->class->sem, however. Is there a locking
> >  mismatch here? But none of the locking code here seems to be changed
> >  in years...
> 
> I think this seems correct.
> 
> Everywhere else where we traverse the struct class->devices list, they
> have down(&class->sem); first and up(&class->sem); afterwards.
> 
> Commit fd04897bb20be29d60f7e426a053545aebeaa61a even has this hunk:
> @@ -177,8 +177,7 @@ struct class {
>         struct list_head        devices;
>         struct list_head        interfaces;
>         struct kset             class_dirs;
> -       struct semaphore        sem;    /* locks both the children and interface
> -
> +       struct semaphore        sem; /* locks children, devices, interfaces */
>         struct class_attribute          * class_attrs;
>         struct class_device_attribute   * class_dev_attrs;
>         struct device_attribute         * dev_attrs;
> 
> 
> So why doesn't block/genhd.c do this too? It seems to me that the
> mutex locking here is simply a remnant of old code that happened to
> not crash in most cases by chance.
> 

Does this crash happen with the conversion to the class iterator
functions (should be in linux-next) as well? They take the class
mutex...
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