linux-kernel - Re: [RFC PATCH v1 27/50] drivers/s390/scsi/zcsp_fc.c: Use prandom_u32

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Date:   Tue, 31 Mar 2020 18:13:21 +0200
From:   Benjamin Block <bblock@...ux.ibm.com>
To:     George Spelvin <lkml@....org>, Steffen Maier <maier@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [RFC PATCH v1 27/50] drivers/s390/scsi/zcsp_fc.c: Use
 prandom_u32_max() for backoff

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@....org>
> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> Cc: Vasily Gorbik <gor@...ux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@...ibm.com>
> Cc: linux-s390@...r.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);

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.

>  }
>  
>  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.

-- 
Best Regards, Benjamin Block  / Linux on IBM Z Kernel Development / IBM Systems
IBM Deutschland Research & Development GmbH    /    https://www.ibm.com/privacy
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