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Subject: Re: [RFC PATCH v1 27/50] drivers/s390/scsi/zcsp_fc.c: Use
 prandom_u32_max() for backoff
To:     Benjamin Block <bblock@linux.ibm.com>,
        George Spelvin <lkml@sdf.org>
Cc:     linux-kernel@vger.kernel.org,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org
References: <202003281643.02SGhHN7015213@sdf.org>
 <20200331161321.GB17507@t480-pf1aa2c2>
From:   Steffen Maier <maier@linux.ibm.com>
Date:   Tue, 31 Mar 2020 18:23:04 +0200
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On 3/31/20 6:13 PM, Benjamin Block wrote:
> On Fri, Nov 29, 2019 at 03:39:41PM -0500, George Spelvin wrote:
>> We don't need crypto-grade random numbers for randomized backoffs.
>>
>> (We could skip the if() if we wanted to rely on the undocumented fact
>> that prandom_u32_max(0) always returns 0.  That would be a net time
>> saving it port_scan_backoff == 0 is rare; if it's common, the if()
>> is false often enough to pay for itself. Not sure which applies here.)
>>
>> Signed-off-by: George Spelvin <lkml@sdf.org>
>> Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Vasily Gorbik <gor@linux.ibm.com>
>> Cc: Christian Borntraeger <borntraeger@de.ibm.com>
>> Cc: linux-s390@vger.kernel.org
>> ---
>>   drivers/s390/scsi/zfcp_fc.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Hello George,
> 
> it would be nice, if you could address the mails to the
> driver-maintainers (`scripts/get_maintainer.pl drivers/s390/scsi/zfcp_fc.c`
> will tell you that this is me and Steffen); I'd certainly have noticed
> it earlier then :-).
> 
>>
>> diff --git a/drivers/s390/scsi/zfcp_fc.c b/drivers/s390/scsi/zfcp_fc.c
>> index b018b61bd168e..d24cafe02708f 100644
>> --- a/drivers/s390/scsi/zfcp_fc.c
>> +++ b/drivers/s390/scsi/zfcp_fc.c
>> @@ -48,7 +48,7 @@ unsigned int zfcp_fc_port_scan_backoff(void)
>>   {
>>   	if (!port_scan_backoff)
>>   		return 0;
>> -	return get_random_int() % port_scan_backoff;
>> +	return prandom_u32_max(port_scan_backoff);

Reviewed-by: Steffen Maier <maier@linux.ibm.com>

> 
> I think the change is fine. You are right, we don't need a crypto nonce
> here.
> 
> I think I'd let the zero-check stand as is, because the internal
> behaviour of prandom_u32_max() is, as you say, undocumented. This is not
> a performance critical code-path for us anyway.

yes, let's keep the extra check as it's intentional and documented user 
interface for zfcp, so better be explicit

> 
>>   }
>>   
>>   static void zfcp_fc_port_scan_time(struct zfcp_adapter *adapter)
>> -- 
>> 2.26.0
>>
> 
> Steffen, do you have any objections? Otherwise I can queue this up -
> minus the somewhat mangled subject - for when we send something next time.
> 


-- 
Mit freundlichen Gruessen / Kind regards
Steffen Maier

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