Re: [PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio ring page migration.

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

Hi Tang,

(2014/02/27 19:40), Tang Chen wrote:
> When doing aio ring page migration, we migrated the page, and update
> ctx->ring_pages[]. Like the following:
> 
> aio_migratepage()
>   |-> migrate_page_copy(new, old)
>   |   ......				/* Need barrier here */
>   |-> ctx->ring_pages[idx] = new
> 
> Actually, we need a memory barrier between these two operations.
> Otherwise, if ctx->ring_pages[] is updated before memory copy due to
> the compiler optimization, other processes may have an opportunity
> to access to the not fully initialized new ring page.
> 
> So add a wmb to synchronize them.
> 
> Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
> Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
> ---
>   fs/aio.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 50c089c..f0ed838 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -327,6 +327,14 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
>   		pgoff_t idx;
>   		spin_lock_irqsave(&ctx->completion_lock, flags);
>   		migrate_page_copy(new, old);
> +
> +		/*
> +		 * Ensure memory copy is finished before updating
> +		 * ctx->ring_pages[]. Otherwise other processes may access to
> +		 * new ring pages which are not fully initialized.
> +		 */
> +		smp_wmb();
> +

If you put smp_wmb() here, you should put smp_rmb() before kmap() in
aio_read_events_ring().

Thanks,
Yasuaki Ishimatsu

>   		idx = old->index;
>   		if (idx < (pgoff_t)ctx->nr_pages) {
>   			/* And only do the move if things haven't changed */
>