[PATCH 2/2] null_blk: fix set->driver_data while setting up tagset

[Nilay Shroff <nilay@linux.ibm.com>]

When setting up a null block device, we initialize a tagset that
includes a driver_data field—typically used by block drivers to
store a pointer to driver-specific data. In the case of null_blk,
this should point to the struct nullb instance.

However, due to recent tagset refactoring in the null_blk driver, we
missed initializing driver_data when creating a shared tagset. As a
result, software queues (ctx) fail to map correctly to new hardware
queues (hctx). For example, increasing the number of submit queues
triggers an nr_hw_queues update, which invokes null_map_queues() to
remap queues. Since set->driver_data is unset, null_map_queues()
fails to map any ctx to the new hctxs, leading to hctx->nr_ctx == 0,
effectively making the hardware queues unusable for I/O.

This patch fixes the issue by ensuring that set->driver_data is properly
initialized to point to the struct nullb during tagset setup.

Fixes: 72ca28765fc4 ("null_blk: refactor tag_set setup")
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/block/null_blk/main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index aa163ae9b2aa..9e1c4ce6fc42 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -1854,13 +1854,14 @@ static int null_init_global_tag_set(void)
 
 static int null_setup_tagset(struct nullb *nullb)
 {
+	nullb->tag_set->driver_data = nullb;
+
 	if (nullb->dev->shared_tags) {
 		nullb->tag_set = &tag_set;
 		return null_init_global_tag_set();
 	}
 
 	nullb->tag_set = &nullb->__tag_set;
-	nullb->tag_set->driver_data = nullb;
 	nullb->tag_set->nr_hw_queues = nullb->dev->submit_queues;
 	nullb->tag_set->queue_depth = nullb->dev->hw_queue_depth;
 	nullb->tag_set->numa_node = nullb->dev->home_node;
-- 
2.50.1