linux-kernel - Re: How to avoid spurious lockdep warnings?

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Message-ID: <47E57546.9030403@goop.org>
Date:	Sat, 22 Mar 2008 14:08:22 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: How to avoid spurious lockdep warnings?

Arjan van de Ven wrote:
> On Thu, 20 Mar 2008 16:02:11 -0700
> Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> In a Xen system, when a new pagetable is about to be put in use it is 
>> "pinned", meaning that each page in the pagetable is registered with
>> the hypervisor.  This is done in arch/x86/xen/mmu.c:pin_page().
>>
>> In order to make this efficient, the hypercalls for pinning are
>> batched, so that multiple pages are submitted at once in a single
>> multicall. While a page is batched pending the hypercall, its
>> corresponding pte_lock is held.
>>
>> This means that the code can end up holding multiple pte locks at
>> once, though it is guaranteed to never try to hold the same lock at
>> once. However, because these locks are in the same lock class, I get
>> a spurious warning from lockdep.  Is there some way I can get rid of
>> this warning?
>>     
>
>
> what's the ordering guarantee between these locks ?
>   

Pagetable virtual address order.  move_ptes can also lock two ptes 
within one pagetable, without regard to order, but they're protected by 
mmap_sem, which is also held during pinning.

    J
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