Fix null pointer error by checking if q->tag_set is null.
Address "block/elevator.c:569 elevator_get_default() error:
we previously assumed 'q->tag_set' could be null (see line 565)"
This change prevents errors by making sure q->tag_set is valid
before accessing its flags.

Signed-off-by: SurajSonawane2415 <surajsonawane0215@gmail.com>
---
 block/elevator.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 4122026b1..9ca32a6bd 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -562,12 +562,14 @@ static inline bool elv_support_iosched(struct request_queue *q)
  */
 static struct elevator_type *elevator_get_default(struct request_queue *q)
 {
-	if (q->tag_set && q->tag_set->flags & BLK_MQ_F_NO_SCHED_BY_DEFAULT)
-		return NULL;
+	if (q->tag_set) {
+		if (q->tag_set->flags & BLK_MQ_F_NO_SCHED_BY_DEFAULT)
+			return NULL;
 
-	if (q->nr_hw_queues != 1 &&
-	    !blk_mq_is_shared_tags(q->tag_set->flags))
-		return NULL;
+		if (q->nr_hw_queues != 1 &&
+			!blk_mq_is_shared_tags(q->tag_set->flags))
+			return NULL;
+	}
 
 	return elevator_find_get(q, "mq-deadline");
 }
-- 
2.34.1

>  static struct elevator_type *elevator_get_default(struct request_queue *q)
>  {
> -	if (q->tag_set && q->tag_set->flags & BLK_MQ_F_NO_SCHED_BY_DEFAULT)
> -		return NULL;
> +	if (q->tag_set) {

q->tag_set can't be NULL for a blk-mq queue.  So you can just remove
the q->tag_set check here and also in elv_support_iosched.

Fix error "block/elevator.c:569 elevator_get_default() error:
we previously assumed 'q->tag_set' could be null (see line 565)".
Since 'q->tag_set' cannot be NULL for blk-mq queues,
remove the unnecessary check in both `elevator_get_default`
and `elv_support_iosched`. This simplifies the logic and
ensures correct assumptions about 'q->tag_set' in blk-mq queues.

Signed-off-by: SurajSonawane2415 <surajsonawane0215@gmail.com>
---
V1 -> V2:
- Remove unnecessary `q->tag_set` check in `elv_support_iosched` function.
- Remove `q->tag_set` check because blk-mq queues always have a valid tag set.

 block/elevator.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index 4122026b1..60497e3b2 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -551,7 +551,7 @@ EXPORT_SYMBOL_GPL(elv_unregister);
 static inline bool elv_support_iosched(struct request_queue *q)
 {
 	if (!queue_is_mq(q) ||
-	    (q->tag_set && (q->tag_set->flags & BLK_MQ_F_NO_SCHED)))
+	    (q->tag_set->flags & BLK_MQ_F_NO_SCHED))
 		return false;
 	return true;
 }
@@ -562,7 +562,7 @@ static inline bool elv_support_iosched(struct request_queue *q)
  */
 static struct elevator_type *elevator_get_default(struct request_queue *q)
 {
-	if (q->tag_set && q->tag_set->flags & BLK_MQ_F_NO_SCHED_BY_DEFAULT)
+	if (q->tag_set->flags & BLK_MQ_F_NO_SCHED_BY_DEFAULT)
 		return NULL;
 
 	if (q->nr_hw_queues != 1 &&
-- 
2.34.1

On Mon, 07 Oct 2024 16:44:16 +0530, SurajSonawane2415 wrote:
> Fix error "block/elevator.c:569 elevator_get_default() error:
> we previously assumed 'q->tag_set' could be null (see line 565)".
> Since 'q->tag_set' cannot be NULL for blk-mq queues,
> remove the unnecessary check in both `elevator_get_default`
> and `elv_support_iosched`. This simplifies the logic and
> ensures correct assumptions about 'q->tag_set' in blk-mq queues.
> 
> [...]

Applied, thanks!

[1/1] block: Fix elevator_get_default() to check for null q->tag_set
      commit: b402328a24ee7193a8ab84277c0c90ae16768126

Best regards,
-- 
Jens Axboe

On 10/7/24 5:14 AM, SurajSonawane2415 wrote:
> Fix error "block/elevator.c:569 elevator_get_default() error:
> we previously assumed 'q->tag_set' could be null (see line 565)".
> Since 'q->tag_set' cannot be NULL for blk-mq queues,
> remove the unnecessary check in both `elevator_get_default`
> and `elv_support_iosched`. This simplifies the logic and
> ensures correct assumptions about 'q->tag_set' in blk-mq queues.

For future patches:

1) Wrap at 72 characters, your commit message is using shorter line
   lengths.

2) Don't both with file numbers, they change all of the time. Digging
   out an old commit with line numbers isn't very useful, you'd have to
   rewind your tree for them to make sense.

I'll commit this, but rewrite the commit message somewhat.

-- 
Jens Axboe