null_blk: fix set->driver_data while setting up tagset

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: 72ca287 ("null_blk: refactor tag_set setup")
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Nilay Shroff <[email protected]>

Author: shroffni

drivers/block/null_blk/main.c

@@ -1856,6 +1856,7 @@ static int null_setup_tagset(struct nullb *nullb)
 {
 	if (nullb->dev->shared_tags) {
 		nullb->tag_set = &tag_set;
+		nullb->tag_set->driver_data = nullb;
 		return null_init_global_tag_set();
 	}