linux-kernel - [Update v2 PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio ring page migration.

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Date:	Wed, 5 Mar 2014 16:17:42 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Tang Chen <tangchen@...fujitsu.com>, <bcrl@...ck.org>
CC:	<viro@...iv.linux.org.uk>, <jmoyer@...hat.com>,
	<kosaki.motohiro@...il.com>, <kosaki.motohiro@...fujitsu.com>,
	<guz.fnst@...fujitsu.com>, <linux-fsdevel@...r.kernel.org>,
	<linux-aio@...ck.org>, <linux-kernel@...r.kernel.org>,
	<miaox@...fujitsu.com>
Subject: [Update v2 PATCH 2/2] aio, mem-hotplug: Add memory barrier to aio
 ring page migration.

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@...fujitsu.com>
Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
---
v2: change smp_rmb() to smp_read_barrier_depends(). Thanks Miao.
---
 fs/aio.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/fs/aio.c b/fs/aio.c
index 50c089c..98c7f2d 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_read_barrier_depends();
+
 		ev = kmap(page);
 		copy_ret = copy_to_user(event + ret, ev + pos,
 					sizeof(*ev) * avail);

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