Re: 2.6.35-rc6-git6: Reported regressions from 2.6.34

From: Tejun Heo
Date: Mon Aug 02 2010 - 12:33:36 EST

Hello, Linus.

On 08/01/2010 08:01 PM, Linus Torvalds wrote:
> This has a proposed patch. I don't know what the status of it is, though. Jens?
>> Bug-Entry :
>> Subject : kernel BUG at fs/block_dev.c:765!
>> Submitter : Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>
>> Date : 2010-07-14 13:52 (19 days old)
>> Message-ID : <20100714135217.GA1797@xxxxxxxxxxxxx>
>> References :
> This one is interesting. And I think I perhaps see where it's coming from.
> bd_start_claiming() (through bd_prepare_to_claim()) has two separate
> success cases: either there was no holder (bd_claiming is NULL) or the
> new holder was already claiming it (bd_claiming == holder).
> Note in particular the case of the holder _already_ holding it. What happens is:
> - bd_start_claiming() succeeds because we had _already_ claimed it
> with the same holder
> - then some error happens, and we call bd_abort_claiming(), which
> does whole->bd_claiming = NULL;
> - the original holder thinks it still holds the bd, but it has been released!
> - a new claimer comes in, and succeeds because bd_claiming is now NULL.
> - we now have two "owners" of the bd, but bd_claiming only points to
> the second one.
> I think bd_start_claiming() needs to do some kind of refcount for the
> nested holder case, and bd_abort_claiming() needs to decrement the
> refcount and only clear the bd_claiming field when it goes down to
> zero.
> I dunno. Maybe there's something else going on, but it does look
> suspicious, and the above would explain the BUG_ON().

Yeah, that definitely sounds plausible. I think the condition check
in bd_prepare_to_claim() should have been "if (whole->bd_claiming)"
instead of "if (whole->bd_claiming && whole->bd_claiming != holder)".
It doesn't make much sense to allow multiple parallel claiming
operations anyway and the comment above already says - "This function
fails if @bdev is already claimed by another holder and waits if
another claiming is in progress."

I'll try to build a test case and verify it.

Thank you.

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