[PATCH BUGFIX] block: make elevator_get robust against cross blk/blk-mq choice

[Paolo Valente]

If, at boot, a legacy I/O scheduler is chosen for a device using blk-mq,
or, viceversa, a blk-mq scheduler is chosen for a device using blk, then
that scheduler is set and initialized without any check, driving the
system into an inconsistent state. This commit addresses this issue by
letting elevator_get fail for these wrong cross choices.

Signed-off-by: Paolo Valente <paolo.vale...@linaro.org>
---
 block/elevator.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 27ff1ed..a25bdd9 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -99,7 +99,8 @@ static void elevator_put(struct elevator_type *e)
        module_put(e->elevator_owner);
 }
 
-static struct elevator_type *elevator_get(const char *name, bool try_loading)
+static struct elevator_type *elevator_get(const char *name, bool try_loading,
+                                         bool mq_ops)
 {
        struct elevator_type *e;
 
@@ -113,6 +114,12 @@ static struct elevator_type *elevator_get(const char 
*name, bool try_loading)
                e = elevator_find(name);
        }
 
+       if (e && (e->uses_mq != mq_ops)) {
+               pr_err("ERROR: attempted to choose %s %s I/O scheduler in 
blk%s",
+                      name, e->uses_mq ? "blk-mq" : "legacy", mq_ops ? "-mq" : 
"");
+               e = NULL;
+       }
+
        if (e && !try_module_get(e->elevator_owner))
                e = NULL;
 
@@ -201,7 +208,7 @@ int elevator_init(struct request_queue *q, char *name)
        q->boundary_rq = NULL;
 
        if (name) {
-               e = elevator_get(name, true);
+               e = elevator_get(name, true, q->mq_ops);
                if (!e)
                        return -EINVAL;
        }
@@ -212,7 +219,7 @@ int elevator_init(struct request_queue *q, char *name)
         * off async and request_module() isn't allowed from async.
         */
        if (!e && *chosen_elevator) {
-               e = elevator_get(chosen_elevator, false);
+               e = elevator_get(chosen_elevator, false, q->mq_ops);
                if (!e)
                        printk(KERN_ERR "I/O scheduler %s not found\n",
                                                        chosen_elevator);
@@ -220,17 +227,20 @@ int elevator_init(struct request_queue *q, char *name)
 
        if (!e) {
                if (q->mq_ops && q->nr_hw_queues == 1)
-                       e = elevator_get(CONFIG_DEFAULT_SQ_IOSCHED, false);
+                       e = elevator_get(CONFIG_DEFAULT_SQ_IOSCHED, false,
+                                        q->mq_ops);
                else if (q->mq_ops)
-                       e = elevator_get(CONFIG_DEFAULT_MQ_IOSCHED, false);
+                       e = elevator_get(CONFIG_DEFAULT_MQ_IOSCHED, false,
+                                        q->mq_ops);
                else
-                       e = elevator_get(CONFIG_DEFAULT_IOSCHED, false);
+                       e = elevator_get(CONFIG_DEFAULT_IOSCHED, false,
+                                        q->mq_ops);
 
                if (!e) {
                        printk(KERN_ERR
                                "Default I/O scheduler not found. " \
                                "Using noop/none.\n");
-                       e = elevator_get("noop", false);
+                       e = elevator_get("noop", false, q->mq_ops);
                }
        }
 
@@ -1051,7 +1061,7 @@ static int __elevator_change(struct request_queue *q, 
const char *name)
                return elevator_switch(q, NULL);
 
        strlcpy(elevator_name, name, sizeof(elevator_name));
-       e = elevator_get(strstrip(elevator_name), true);
+       e = elevator_get(strstrip(elevator_name), true, q->mq_ops);
        if (!e) {
                printk(KERN_ERR "elevator: type %s not found\n", elevator_name);
                return -EINVAL;
-- 
2.10.0