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    SubjectRe: 2.6.35-rc6-git6: Reported regressions from 2.6.34
    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 <>
    >> Date : 2010-07-14 13:52 (19 days old)
    >> Message-ID : <>
    >> 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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