linux-kernel - Re: Question about x86/mm/gup.c's use of disabled interrupts

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Message-ID: <49C16A48.4090303@redhat.com>
Date:	Wed, 18 Mar 2009 23:40:24 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts

Jeremy Fitzhardinge wrote:
>>> Disabling the interrupt will prevent the tlb flush IPI from coming 
>>> in and flushing this cpu's tlb, but I don't see how it will prevent 
>>> some other cpu from actually updating the pte in the pagetable, 
>>> which is what we're concerned about here.  
>>
>> The thread that cleared the pte holds the pte lock and is now waiting 
>> for the IPI.  The thread that wants to update the pte will wait for 
>> the pte lock, thus also waits on the IPI and gup_fast()'s 
>> local_irq_enable().  I think.
>
> But hasn't it already done the pte update at that point?
>
> (I think this conversation really is moot because the kernel never 
> does P->P pte updates any more; its always P->N->P.)

I thought you were concerned about cpu 0 doing a gup_fast(), cpu 1 doing 
P->N, and cpu 2 doing N->P.  In this case cpu 2 is waiting on the pte lock.

>>> Is this the only reason to disable interrupts?  
>>
>> Another comment says it also prevents pagetable teardown.
>
> We could take a reference to the mm to get the same effect, no?
>

Won't stop munmap().

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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