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Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting
 method to skip serializing
To: Leonardo Bras <leonardo@linux.ibm.com>, <linuxppc-dev@lists.ozlabs.org>,
 <linux-kernel@vger.kernel.org>
References: <20190920195047.7703-1-leonardo@linux.ibm.com>
 <20190920195047.7703-12-leonardo@linux.ibm.com>
From: John Hubbard <jhubbard@nvidia.com>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 9/20/19 12:50 PM, Leonardo Bras wrote:
> Skips slow part of serialize_against_pte_lookup if there is no running
> lockless pagetable walk.
> 
> Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> ---
>  arch/powerpc/mm/book3s64/pgtable.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> index 13239b17a22c..41ca30269fa3 100644
> --- a/arch/powerpc/mm/book3s64/pgtable.c
> +++ b/arch/powerpc/mm/book3s64/pgtable.c
> @@ -95,7 +95,8 @@ static void do_nothing(void *unused)
>  void serialize_against_pte_lookup(struct mm_struct *mm)
>  {
>  	smp_mb();
> -	smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
> +	if (running_lockless_pgtbl_walk(mm))
> +		smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);

Hi,

If you do this, then you are left without any synchronization. So it will
have race conditions: a page table walk could begin right after the above
check returns "false", and then code such as hash__pmdp_huge_get_and_clear()
will continue on right away, under the false assumption that it has let
all the current page table walks complete.

The current code uses either interrupts or RCU to synchronize, and in
either case, you end up scheduling something on each CPU. If you remove
that entirely, I don't see anything left. ("Pure" atomic counting is not
a synchronization technique all by itself.)

thanks,
-- 
John Hubbard
NVIDIA

>  }
>  
>  void start_lockless_pgtbl_walk(struct mm_struct *mm)
>