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

[Yasuaki Ishimatsu]

(2014/02/27 23:57), Benjamin LaHaise wrote:
> On Thu, Feb 27, 2014 at 06:40:16PM +0800, 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.
>
> The smp_wmb() is not needed after you added the taking of ctx->completion_lock
> lock since all accesses to ring_pages is then protected by the spinlock.
> Why are you adding this then?  Or have you missed adding the lock somewhere?

Pleaes see following thread. It's a updated patch.

https://lkml.org/lkml/2014/2/27/237

aio_read_events_ring() accesses ring_pages without locking ctx-completion_lock.
If copying old page'date to new page runs after new page was set to ctx->ring_pages
by changing order, aio_read_events_ring() may not work correctly.

So smp_wmb() and smp_rmb() is needed.

Thanks,
Yasuaki Ishimatsu

> Also, if you've changed the patch, it is customary to add a "v2" somewhere in
> the patch title so that I have some idea what version of the patch should be
> applied.
>
> 		-ben



> > Reported-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
> > Signed-off-by: Tang Chen <tangchen@xxxxxxxxxxxxxx>
> > ---
> >   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();
> > +
> >   		idx = old->index;
> >   		if (idx < (pgoff_t)ctx->nr_pages) {
> >   			/* And only do the move if things haven't changed */
> > --
> > 1.8.3.1




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