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

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Date:	Wed, 18 Mar 2009 14:23:38 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
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

Avi Kivity wrote:
> 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.)

>> 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?

>> Also, assuming that disabling the interrupt is enough to get the 
>> guarantees we need here, there's a Xen problem because we don't use 
>> IPIs for cross-cpu tlb flushes (well, it happens within Xen).  I'll 
>> have to think a bit about how to deal with that, but I'm thinking 
>> that we could add a per-cpu "tlb flushes blocked" flag, and maintain 
>> some kind of per-cpu deferred tlb flush count so we can get around to 
>> doing the flush eventually.
>
> I was thinking about adding a hypercall for cross-vcpu tlb flushes.  
> Guess I'll wait for you to clear up all the issues first.

Typical...

    J

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