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SubjectRe: [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@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);

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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