[PATCH 3.16 036/357] aio: add missing smp_rmb() in read_events_ring

[Greg Kroah-Hartman]

3.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jeff Moyer <jmoyer@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Benjamin LaHaise <bcrl@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/aio.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1134,6 +1134,12 @@ static long aio_read_events_ring(struct
 	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)


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/