Subject: Re: [patch 1/1] powerpc: add rcu_read_lock() to gup_fast() implementation

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 28 Apr 2010 14:26:18 +1000

On Tue, 2010-04-27 at 14:10 -0700, [email-masked] wrote:
> From: Peter Zijlstra <[email-masked]>
> 
> 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.

Nah, that's not right. The right fix is to use call_rcu_sched() from
the powerpc page table freeing code. Please drop.

Cheers,
Ben.

> Signed-off-by: Peter Zijlstra <[email-masked]>
> Requested-by: Paul E. McKenney <[email-masked]>
> Cc: Nick Piggin <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Reviewed-by: Rik van Riel <[email-masked]>
> Signed-off-by: Andrew Morton <[email-masked]>
> ---
> 
>  arch/powerpc/mm/gup.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff -puN arch/powerpc/mm/gup.c~powerpc-add-rcu_read_lock-to-gup_fast-implementation arch/powerpc/mm/gup.c
> --- a/arch/powerpc/mm/gup.c~powerpc-add-rcu_read_lock-to-gup_fast-implementation
> +++ a/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);
>  
> _