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

[Yasuaki Ishimatsu]

(2014/03/04 14:35), Miao Xie wrote:
> 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@xxxxxxxxxxxxxx>
>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
>>
>> ---
>>   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

Thank you for your comment.
I'll update soon.

Thanks,
Yasauaki Ishimatsu


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


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