Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Avi Kivity <avi@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
	Rik van Riel <riel@redhat.com>, Ingo Molnar <mingo@elte.hu>,
	akpm@linux-foundation.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	David Miller <davem@davemloft.net>,
	Hugh Dickins <hugh.dickins@tiscali.co.uk>,
	Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@suse.de>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation
Date: Tue, 13 Apr 2010 11:05:31 +1000	[thread overview]
Message-ID: <1271120731.13059.6.camel@pasglop> (raw)
In-Reply-To: <20100408192722.643778654@chello.nl>

On Thu, 2010-04-08 at 21:17 +0200, Peter Zijlstra wrote:
> plain text document attachment (powerpc-gup_fast-rcu.patch)
> The powerpc page table freeing relies on the fact that IRQs hold off
> an RCU grace period, this is currently true for all existing RCU
> implementations but is not an assumption Paul wants to support.
> 
> Therefore, also take the RCU read lock along with disabling IRQs to
> ensure the RCU grace period does at least cover these lookups.

There's a few other places that need a similar fix then. The hash page
code for example. All the C cases should end up calling the
find_linux_pte() helper afaik, so we should be able to stick the lock in
there (and the hugetlbfs variant, find_linux_pte_or_hugepte()).

However, we also have cases of tight asm code walking the page tables,
such as the tlb miss handler on embedded processors. I don't see how I
could do that there. IE. I only have a handful of registers to play
with, no stack, etc...

So we might have to support the interrupt assumption, at least in some
form, with those guys...

Cheers,
Ben. 

> Requested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Nick Piggin <npiggin@suse.de>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> ---
>  arch/powerpc/mm/gup.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> Index: linux-2.6/arch/powerpc/mm/gup.c
> ===================================================================
> --- linux-2.6.orig/arch/powerpc/mm/gup.c
> +++ linux-2.6/arch/powerpc/mm/gup.c
> @@ -142,6 +142,7 @@ int get_user_pages_fast(unsigned long st
>  	 * So long as we atomically load page table pointers versus teardown,
>  	 * we can follow the address down to the the page and take a ref on it.
>  	 */
> +	rcu_read_lock();
>  	local_irq_disable();
>  
>  	pgdp = pgd_offset(mm, addr);
> @@ -162,6 +163,7 @@ int get_user_pages_fast(unsigned long st
>  	} while (pgdp++, addr = next, addr != end);
>  
>  	local_irq_enable();
> +	rcu_read_unlock();
>  
>  	VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
>  	return nr;
> @@ -171,6 +173,7 @@ int get_user_pages_fast(unsigned long st
>  
>  slow:
>  		local_irq_enable();
> +		rcu_read_unlock();
>  slow_irqon:
>  		pr_devel("  slow path ! nr = %d\n", nr);
>  
> 