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

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

On 7/21/25 6:34 PM, Damien Le Moal wrote:
> On 2025/07/20 20:35, 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")
>> 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;
>> +
> 
> How can this be correct since the tag_set pointer is initialized below ?
> 
>>  	if (nullb->dev->shared_tags) {
>>  		nullb->tag_set = &tag_set;
> 
> Shouldn't you add:
> 
> 		nullb->tag_set->driver_data = nullb;
> 
> here instead ?
> 
>>  		return null_init_global_tag_set();
>>  	}

Oh yes good catch! I'll fix this in the next patchset.
My bad.. :(

Thanks,
--Nilay