Re: [RESEND v2 PATCH 1/2] aio, memory-hotplug: Fix confliction when migrating and accessing ring pages.

[Benjamin LaHaise]

Hello Tang,

On Wed, Mar 12, 2014 at 01:25:26PM +0800, Tang Chen wrote:
... <snip> ...

> >Another spot is in
> >aio_read_events_ring() where head and tail are fetched from the ring 
> >without
> >any locking.  I also fear we'll be introducing new performance issues with
> >all the additonal spinlock bouncing, despite the fact that is only ever
> >needed for migration.  I'm going to continue looking into this today and
> >will try to send out a followup to this email later.
> 
> In the beginning of aio_read_events_ring(), it reads head and tail, not 
> write.
> So even if ring pages are migrated, the contents of the pages will not 
> be changed.
> So reading it is OK, from old page or from the new page, I think.

Your assumption that reading it is okay is incorrect.  Since we do not have 
a reference on the page at that point, it is possible that the read of the 
page takes place after the page has been freed and allocated to another part 
of the kernel.  This would result in the read returning invalid information.

		-ben
-- 
"Thought is the essence of where you are now."
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