linux-kernel - Re: [PATCH 1/2] scsi: core: cleanup scsi_dev_queue

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Date:   Sun, 24 Sep 2023 14:27:46 +0800
From:   Wenchao Hao <haowenchao2@...wei.com>
To:     Damien Le Moal <dlemoal@...nel.org>
CC:     "James E . J . Bottomley" <jejb@...ux.ibm.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <louhongxiang@...wei.com>
Subject: Re: [PATCH 1/2] scsi: core: cleanup scsi_dev_queue_ready()

On 2023/9/22 20:50, Damien Le Moal wrote:
> On 2023/09/22 2:38, Wenchao Hao wrote:
>> This is just a cleanup for scsi_dev_queue_ready() to avoid
>> redundant goto and if statement, it did not change the origin
>> logic.
>>
>> Signed-off-by: Wenchao Hao <haowenchao2@...wei.com>
>> ---
>>   drivers/scsi/scsi_lib.c | 35 ++++++++++++++++++-----------------
>>   1 file changed, 18 insertions(+), 17 deletions(-)
>>
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index ca5eb058d5c7..f3e388127dbd 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -1254,28 +1254,29 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
>>   	int token;
>>   
>>   	token = sbitmap_get(&sdev->budget_map);
>> -	if (atomic_read(&sdev->device_blocked)) {
>> -		if (token < 0)
>> -			goto out;
>> +	if (token < 0)
>> +		return -1;
> 
> This is changing how this function works...
>

I don't think so...
The origin function flow:

static inline int scsi_dev_queue_ready(struct request_queue *q,
				  struct scsi_device *sdev)
{
	...
	token = sbitmap_get(&sdev->budget_map);
	if (atomic_read(&sdev->device_blocked)) {
		if (token < 0)
			goto out;
	}
	return token;
out:
	return -1
}

If the token is less than 0, it would always return -1. So we can found
it's not necessary to check token after atomic_read().
  
>>   
>> -		if (scsi_device_busy(sdev) > 1)
>> -			goto out_dec;
>> +	/*
>> +	 * device_blocked is not set at mostly time, so check it first
>> +	 * and return token when it is not set.
>> +	 */
>> +	if (!atomic_read(&sdev->device_blocked))
>> +		return token;
> 
> ...because you reversed the tests order.

As explained in comment, the device_blocked is not set at mostly time,
so when it's not set, just return the token.

>>   
>> -		/*
>> -		 * unblock after device_blocked iterates to zero
>> -		 */
>> -		if (atomic_dec_return(&sdev->device_blocked) > 0)
>> -			goto out_dec;
>> -		SCSI_LOG_MLQUEUE(3, sdev_printk(KERN_INFO, sdev,
>> -				   "unblocking device at zero depth\n"));
>> +	/*
>> +	 * unblock after device_blocked iterates to zero
>> +	 */
>> +	if (scsi_device_busy(sdev) > 1 ||
>> +	    atomic_dec_return(&sdev->device_blocked) > 0) {
> 
> And here too, you are changing how the function works. The atomic_dec may not be
> done if the first condition is true.
> 

the origin flow would not call atomic_dec_return() too when condition
"scsi_device_busy(sdev) > 1" is true like following:

static inline int scsi_dev_queue_ready(struct request_queue *q,
				  struct scsi_device *sdev)
{
	...
	if (atomic_read(&sdev->device_blocked)) {
		if (scsi_device_busy(sdev) > 1)
			goto out_dec;

		/*
		 * unblock after device_blocked iterates to zero
		 */
		if (atomic_dec_return(&sdev->device_blocked) > 0)
			goto out_dec;
		SCSI_LOG_MLQUEUE(3, sdev_printk(KERN_INFO, sdev,
				   "unblocking device at zero depth\n"));
	}
	return token;
out_dec:
	if (token >= 0)
		sbitmap_put(&sdev->budget_map, token);
out:
	return -1;
}

Here is the function before and after my change:

The old function:
static inline int scsi_dev_queue_ready(struct request_queue *q,
				  struct scsi_device *sdev)
{
	int token;

	token = sbitmap_get(&sdev->budget_map);
	if (token < 0)
		return -1;

	/*
	 * device_blocked is not set at mostly time, so check it first
	 * and return token when it is not set.
	 */
	if (!atomic_read(&sdev->device_blocked))
		return token;

	/*
	 * unblock after device_blocked iterates to zero
	 */
	if (scsi_device_busy(sdev) > 1 ||
	    atomic_dec_return(&sdev->device_blocked) > 0) {
		sbitmap_put(&sdev->budget_map, token);
		return -1;
	}

	SCSI_LOG_MLQUEUE(3, sdev_printk(KERN_INFO, sdev,
			 "unblocking device at zero depth\n"));

	return token;
}

Function after my change:

static inline int scsi_dev_queue_ready(struct request_queue *q,
				  struct scsi_device *sdev)
{
	int token;

	token = sbitmap_get(&sdev->budget_map);
	if (token < 0)
		return -1;

	/*
	 * device_blocked is not set at mostly time, so check it first
	 * and return token when it is not set.
	 */
	if (!atomic_read(&sdev->device_blocked))
		return token;

	/*
	 * unblock after device_blocked iterates to zero
	 */
	if (scsi_device_busy(sdev) > 1 ||
	    atomic_dec_return(&sdev->device_blocked) > 0) {
		sbitmap_put(&sdev->budget_map, token);
		return -1;
	}

	SCSI_LOG_MLQUEUE(3, sdev_printk(KERN_INFO, sdev,
			 "unblocking device at zero depth\n"));

	return token;
}

I think it looks more clear and readable than before.


>> +		sbitmap_put(&sdev->budget_map, token);
>> +		return -1;
>>   	}
>>   
>> +	SCSI_LOG_MLQUEUE(3, sdev_printk(KERN_INFO, sdev,
>> +			 "unblocking device at zero depth\n"));
>> +
>>   	return token;
>> -out_dec:
>> -	if (token >= 0)
>> -		sbitmap_put(&sdev->budget_map, token);
>> -out:
>> -	return -1;
>>   }
>>   
>>   /*
>