Re: [PATCH 2/2] aio: fix the confliction of read events and migrating ring page
From: Gu Zheng
Date: Thu Mar 20 2014 - 22:05:48 EST
On 03/21/2014 12:30 AM, Benjamin LaHaise wrote:
> On Thu, Mar 20, 2014 at 10:32:07AM -0400, Dave Jones wrote:
>> On Thu, Mar 20, 2014 at 01:46:25PM +0800, Gu Zheng wrote:
>> > diff --git a/fs/aio.c b/fs/aio.c
>> > index 88ad40c..e353085 100644
>> > --- a/fs/aio.c
>> > +++ b/fs/aio.c
>> > @@ -319,6 +319,9 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
>> > ctx->ring_pages[old->index] = new;
>> > spin_unlock_irqrestore(&ctx->completion_lock, flags);
>> > + /* Ensure read event is completed before putting old page */
>> > + mutex_lock(&ctx->ring_lock);
>> > + mutex_unlock(&ctx->ring_lock);
>> > put_page(old);
>> > return rc;
>> This looks a bit weird. Would using a completion work here ?
> Nope. This is actually the most elegant fix I've seen for this approach,
> as everything else has relied on adding additional spin locks (which only
> end up being needed in the migration case) around access to the ring_pages
> on the reader side. That said, this patch is not a complete solution to
> the problem, as the update of the ring's head pointer could still get lost
> with this patch. I think the right thing is just taking the ring_lock
> mutex over the entire page migration operation. That should be safe, as
> nowhere else is the ring_lock mutex nested with any other locks.
This one is based on linux-next which has merged the following patch:
Author: Tang Chen <tangchen@xxxxxxxxxxxxxx>
Date: Mon Mar 10 16:15:33 2014 +0800
aio, memory-hotplug: Fix confliction when migrating and accessing ring pages.
With this patch, the update of the ring's head pointer is safe because it is protected
by completion_lock, so we do not need to enlarge the ring_lock protection region.
And on the other side, if we take the ring_lock over the entire page migration
operation, reading events will be affected if the page migration is going.
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