linux-kernel - Re: [PATCH] lockdep: lock_set_subclass

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Date:	Fri, 1 Aug 2008 20:59:08 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Miller <davem@...emloft.net>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] lockdep: lock_set_subclass - reset a held lock's subclass

On Fri, 1 Aug 2008, Jeremy Fitzhardinge wrote:
> 
> I have a function traversing a pagetable in vaddr order (low to high), taking
> pte locks as it builds up batches of pte page updates.  When the batch is
> issued, it releases all the locks, and won't end up holding more than ~16 at a
> time.
> 
> So, I think this is OK.  There are no internal lock ordering issues, and I
> don't think there'll be any bad interactions from someone trying to take pte
> locks for two separate pagetables.  I don't think there's anyone else trying
> to take more than one pte lock at once, but if there were "lock low vaddr then
> high" seems like a reasonable locking rule (or more precisely "lowest" to deal
> with the case of a pte page being aliased at multiple vaddrs).

Please check the spin_lock_nested() in move_ptes() in mm/mremap.c.

If you have down_write(&mm->mmap_sem) then you should be safe,
but may need to do something to placate lockdep.  If you don't
have down_write(&mm->mmap_sem), then I think you're in trouble?

Not a big deal, the move_ptes() locking can be adjusted to suit
your rule, it was just easier to do it the way it is at the time.

Hugh
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