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

[Miao Xie <miaox@cn.fujitsu.com>]

On 	thu, 27 Feb 2014 21:44:23 +0900, Yasuaki Ishimatsu 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 and rmb to synchronize them.
> 
> Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
> 
> ---
>  fs/aio.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 50c089c..8d9b82b 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();
> +
>  		idx = old->index;
>  		if (idx < (pgoff_t)ctx->nr_pages) {
>  			/* And only do the move if things haven't changed */
> @@ -1074,6 +1082,12 @@ static long aio_read_events_ring(struct kioctx *ctx,
>  		page = ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE];
>  		pos %= AIO_EVENTS_PER_PAGE;
> 
> +		/*
> +		 * Ensure that the page's data was copied from old one by
> +		 * aio_migratepage().
> +		 */
> +		smp_rmb();
> +

smp_read_barrier_depends() is better.

"One could place an A smp_rmb() primitive between the pointer fetch and
 dereference. However, this imposes unneeded overhead on systems (such as
 i386, IA64, PPC, and SPARC) that respect data dependencies on the read side.
 A smp_read_barrier_depends() primitive has been added to the Linux 2.6 kernel
 to eliminate overhead on these systems."
		-- From Chapter 7.1 of <Memory Barriers: a Hardware View for Software Hackers>
		   Written by Paul E. McKenney

Thanks
Miao

>  		ev = kmap(page);
>  		copy_ret = copy_to_user(event + ret, ev + pos,
>  					sizeof(*ev) * avail);
> 
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