[PATCH 3.16.y-ckt 162/254] block: Fix computation of merged request priority

[Luis Henriques]

3.16.7-ckt2 -stable review patch.  If anyone has any objections, please let me know.

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

From: Jan Kara <jack at suse.cz>

commit ece9c72accdc45c3a9484dacb1125ce572647288 upstream.

Priority of a merged request is computed by ioprio_best(). If one of the
requests has undefined priority (IOPRIO_CLASS_NONE) and another request
has priority from IOPRIO_CLASS_BE, the function will return the
undefined priority which is wrong. Fix the function to properly return
priority of a request with the defined priority.

Fixes: d58cdfb89ce0c6bd5f81ae931a984ef298dbda20
Signed-off-by: Jan Kara <jack at suse.cz>
Reviewed-by: Jeff Moyer <jmoyer at redhat.com>
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 block/ioprio.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/block/ioprio.c b/block/ioprio.c
index e50170ca7c33..31666c92b46a 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -157,14 +157,16 @@ out:
 
 int ioprio_best(unsigned short aprio, unsigned short bprio)
 {
-	unsigned short aclass = IOPRIO_PRIO_CLASS(aprio);
-	unsigned short bclass = IOPRIO_PRIO_CLASS(bprio);
+	unsigned short aclass;
+	unsigned short bclass;
 
-	if (aclass == IOPRIO_CLASS_NONE)
-		aclass = IOPRIO_CLASS_BE;
-	if (bclass == IOPRIO_CLASS_NONE)
-		bclass = IOPRIO_CLASS_BE;
+	if (!ioprio_valid(aprio))
+		aprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM);
+	if (!ioprio_valid(bprio))
+		bprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM);
 
+	aclass = IOPRIO_PRIO_CLASS(aprio);
+	bclass = IOPRIO_PRIO_CLASS(bprio);
 	if (aclass == bclass)
 		return min(aprio, bprio);
 	if (aclass > bclass)
-- 
2.1.0