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

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Date:	Mon, 9 Jun 2008 20:09:59 -0700
From:	Greg KH <gregkh@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	"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, Jun 09, 2008 at 09:15:40AM -0700, Linus Torvalds wrote:
> 
> 
> On Mon, 9 Jun 2008, Ingo Molnar wrote:
> > 
> > ah. I suspect that explains the sporadic nature as well: normally there 
> > is 'some' object at the list address, just with an invalid type.
> 
> Yes. It could cause two kinds of problems:
> 
>  - it might end up returning the wrong 'dev_t'. This is unlikely, since we 
>    only have two cases: the working whole-disk case, and the case where we 
>    find a partition.
> 
>    But if we find a partition, we'd still get the right dev_t *most* of 
>    the time, because we'd first get called with "part=0", and then we have 
> 
> 	if (part < disk->minors)
> 		devt = MKDEV(MAJOR(dev->devt),
> 			MINOR(dev->devt) + part);
> 	break;
> 
>    where we would only fail if that conditional statement would be untrue 
>    (and then we'd incorrectly return MKDEV(0,0)). Otherwise, 'devt' ends 
>    up being correct anyway.
> 
>    So one effect of this bug would be that it would use the random 
>    "disk->minors" value to either return the right devt, or return one 
>    that is all zeroes. But if we return the all-zeroes case, then 
>    init/do_mounts.c will just try again, this time with the numbers 
>    removed, and now it wouldn't hit the "strcmp()" on any partition, and 
>    the next time around it would find a disk and work again.
> 
>    So this is a bug, but it's one that essentially is hidden by the 
>    caller.
> 
>  - The other alternative is that the bogus "disk->minors" thing would 
>    cause a page fault. This would only happen if the partition allocation 
>    was the first thing in a page, and the previous page was unused, and 
>    you had DEBUG_PAGEALLOC enabled.
> 
>    This is obviously the case you saw.
> 
> My trivial fix makes it ignore partitions entirely.
> 
> We *could* (and perhaps should) do something slightly more involved 
> instead, which actually uses a partition if it's there). Like this. That 
> would avoid my one nagging worry (that some clever usage makes partitions 
> with a different numbering or without a base block device).
> 
> And this is all still ignoring the locking issue, of course. It would be 
> trivial to just remove the block_class_lock, and change
> 
> 	mutex_[un]lock(&block_class_lock);
> 
> into
> 
> 	down|up(&block_class.sem);

The locking for struct class has turned into a mutex in the -next tree
already, but I have left the block_class_lock alone for the moment.

Now that I have also cleaned up the places in the /proc files where we
grabbed it, I think it might be safe to remove, I'll poke at that
tomorrow.

thanks,

greg k-h
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