Re: [PATCH 2/2] scsi: scsi_debug: fix sparse lock warnings in sdebug_blk_mq_poll()

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 2022/02/28 4:05, Douglas Gilbert wrote:
> On 2022-02-25 03:45, Damien Le Moal wrote:
>> The use of the locked boolean variable to control locking and unlocking
>> of the qc_lock of struct sdebug_queue confuses sparse, leading to a
>> warning about an unexpected unlock. Simplify the qc_lock lock/unlock
>> handling code of this function to avoid this warning by removing the
>> locked boolean variable.
> 
> See below.
> 
>>
>> Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
>> ---
>>   drivers/scsi/scsi_debug.c | 19 +++++++++----------
>>   1 file changed, 9 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
>> index f4e97f2224b2..acb32f3e38eb 100644
>> --- a/drivers/scsi/scsi_debug.c
>> +++ b/drivers/scsi/scsi_debug.c
>> @@ -7509,7 +7509,6 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
>>   {
>>   	bool first;
>>   	bool retiring = false;
>> -	bool locked = false;
>>   	int num_entries = 0;
>>   	unsigned int qc_idx = 0;
>>   	unsigned long iflags;
>> @@ -7525,18 +7524,17 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
>>   	if (qc_idx >= sdebug_max_queue)
>>   		return 0;
>>   
>> +	spin_lock_irqsave(&sqp->qc_lock, iflags);
>> +
>>   	for (first = true; first || qc_idx + 1 < sdebug_max_queue; )   {
>> -		if (!locked) {
>> -			spin_lock_irqsave(&sqp->qc_lock, iflags);
>> -			locked = true;
>> -		}
>>   		if (first) {
>>   			first = false;
>>   			if (!test_bit(qc_idx, sqp->in_use_bm))
>>   				continue;
>> -		} else {
>> -			qc_idx = find_next_bit(sqp->in_use_bm, sdebug_max_queue, qc_idx + 1);
>>   		}
>> +
>> +		qc_idx = find_next_bit(sqp->in_use_bm, sdebug_max_queue,
>> +				       qc_idx + 1);
> 
> The original logic is wrong or the above line is wrong. find_next_bit() is not
> called on the first iteration in the original, but it is with this patch.
> 
>>   		if (qc_idx >= sdebug_max_queue)
>>   			break;
>>   
>> @@ -7586,14 +7584,15 @@ static int sdebug_blk_mq_poll(struct Scsi_Host *shost, unsigned int queue_num)
>>   		}
>>   		WRITE_ONCE(sd_dp->defer_t, SDEB_DEFER_NONE);
>>   		spin_unlock_irqrestore(&sqp->qc_lock, iflags);
>> -		locked = false;
>>   		scsi_done(scp); /* callback to mid level */
>>   		num_entries++;
>> +		spin_lock_irqsave(&sqp->qc_lock, iflags);
>>   		if (find_first_bit(sqp->in_use_bm, sdebug_max_queue) >= sdebug_max_queue)
>>   			break;	/* if no more then exit without retaking spinlock */
> 
> See that comment on the line above? That is the reason for the guard variable.
> Defying that comment, the modified code does a superfluous spinlock irqsave
> and irqrestore.

Rechecking this, there is one point that is bothering me: is it OK to have the
find_first_bit() outside of the sqp lock ? If not, then this is a bug and the
extra lock/unlock that my patch add is a fix...

> 
> Sparse could be taken as a comment on the amount of grey matter that tool has.
> 
> 
>>   	}
>> -	if (locked)
>> -		spin_unlock_irqrestore(&sqp->qc_lock, iflags);
>> +
>> +	spin_unlock_irqrestore(&sqp->qc_lock, iflags);
>> +
>>   	if (num_entries > 0)
>>   		atomic_add(num_entries, &sdeb_mq_poll_count);
>>   	return num_entries;
> 
> Locking issues are extremely difficult to analyze via a unified diff of
> the function. A copy of the original function is required to make any
> sense of it.
> 
> Doug Gilbert
> 
> 


-- 
Damien Le Moal
Western Digital Research