[3.13.y.z extended stable] Patch "aio: add missing smp_rmb() in read_events_ring" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    aio: add missing smp_rmb() in read_events_ring

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 445fa71f4a996952f95ef3e55c500566d0512f12 Mon Sep 17 00:00:00 2001
From: Jeff Moyer <jmoyer at redhat.com>
Date: Tue, 2 Sep 2014 13:17:00 -0400
Subject: aio: add missing smp_rmb() in read_events_ring

commit 2ff396be602f10b5eab8e73b24f20348fa2de159 upstream.

We ran into a case on ppc64 running mariadb where io_getevents would
return zeroed out I/O events.  After adding instrumentation, it became
clear that there was some missing synchronization between reading the
tail pointer and the events themselves.  This small patch fixes the
problem in testing.

Thanks to Zach for helping to look into this, and suggesting the fix.

Signed-off-by: Jeff Moyer <jmoyer at redhat.com>
Signed-off-by: Benjamin LaHaise <bcrl at kvack.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/aio.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/aio.c b/fs/aio.c
index a0d9e43..4555755 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1051,6 +1051,12 @@ static long aio_read_events_ring(struct kioctx *ctx,
 	tail = ring->tail;
 	kunmap_atomic(ring);

+	/*
+	 * Ensure that once we've read the current tail pointer, that
+	 * we also see the events that were stored up to the tail.
+	 */
+	smp_rmb();
+
 	pr_debug("h%u t%u m%u\n", head, tail, ctx->nr_events);

 	if (head == tail)
--
1.9.1