Re: [RFC PATCH v1 27/50] drivers/s390/scsi/zcsp_fc.c: Use prandom_u32_max() for backoff

[Steffen Maier]

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@xxxxxxx>
> > Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> > Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> > Cc: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > Cc: linux-s390@xxxxxxxxxxxxxxx
> > ---
> >   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@xxxxxxxxxxxxx>

> 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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