On 7/21/25 11:04 PM, Nilay Shroff wrote:
> 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")
> Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
> Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
> ---
> drivers/block/null_blk/main.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
> index aa163ae9b2aa..fbae0427263d 100644
> --- a/drivers/block/null_blk/main.c
> +++ b/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;
This looks better, but in the end, why is this even needed ? Since this is a
shared tagset, multiple nullb devices can use that same tagset, so setting the
driver_data pointer to this device only seems incorrect.
Checking the code, the only function that makes use of this pointer is
null_map_queues(), which correctly test for private_data being NULL.
So why do we need this ? Isn't your patch 1/2 enough to fix the crash you got ?
> return null_init_global_tag_set();
> }
>
--
Damien Le Moal
Western Digital Research
