From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932900AbcKQIMr (ORCPT ); Thu, 17 Nov 2016 03:12:47 -0500 Received: from mx2.suse.de ([195.135.220.15]:50125 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750746AbcKQIMg (ORCPT ); Thu, 17 Nov 2016 03:12:36 -0500 Date: Thu, 17 Nov 2016 09:12:29 +0100 From: Johannes Thumshirn To: Arnd Bergmann Cc: Bart Van Assche , "jejb@linux.vnet.ibm.com" , "hch@infradead.org" , "linux-kernel@vger.kernel.org" , "hare@suse.de" , "martin.petersen@oracle.com" , "linux-scsi@vger.kernel.org" Subject: Re: [PATCH] libfc: fix seconds_since_last_reset miscalculation Message-ID: <20161117081229.bie344dn55jekdxl@linux-x5ow.site> References: <1478594694-98847-1-git-send-email-jthumshirn@suse.de> <1479221415.3426.3.camel@sandisk.com> <20161115150531.73jvxfclzzbtpfal@linux-x5ow.site> <4107905.14NieVZ4Uo@wuerfel> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4107905.14NieVZ4Uo@wuerfel> User-Agent: Mutt/1.6.2 (2016-07-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 16, 2016 at 05:58:14PM +0100, Arnd Bergmann wrote: > On Tuesday, November 15, 2016 4:05:31 PM CET Johannes Thumshirn wrote: > > On Tue, Nov 15, 2016 at 02:50:17PM +0000, Bart Van Assche wrote: > > > On Tue, 2016-11-15 at 10:18 +0100, Johannes Thumshirn wrote: > > > > On Tue, Nov 08, 2016 at 03:04:43PM +0000, Bart Van Assche wrote: > > > > > I think the above code will miscalculate seconds_since_last_reset > > > > > if > > > > > 'jiffies' wraps around after an lport has been created and before > > > > > seconds_since_last_reset is computed. Shouldn't > > > > > seconds_since_last_reset > > > > > be computed as follows? > > > > > > > > > > fc_stats->seconds_since_last_reset = (jiffies - boot_time) / > > > > > HZ; > > > > > > > > But what happens when jiffies - boot_time becomes negative? Then we > > > > reintroduce the bug again and have 'fcoeadm -s' show weird values. > > > > > > Hello Johannes, > > > > > > If your concern is about 'jiffies' wrapping around on 32-bit systems > > > then you should use get_jiffies_64(). get_jiffies_64() - boot_time > > > can't become negative. It namely takes several million years before a > > > 64-bit HZ counter wraps around. > > > > You're right. I'll respin using get_jiffies_64() and resent once it is tested. > > Sorry for the bug I introduced and for not noticing this thread earlier. > Looking at this again now, I think it's clear that the bug was simply > mixing up the left and right side of the subtraction, the simple fix > would be > > diff --git a/drivers/scsi/libfc/fc_lport.c b/drivers/scsi/libfc/fc_lport.c > index 2d3133f62463..fe643f2195f0 100644 > --- a/drivers/scsi/libfc/fc_lport.c > +++ b/drivers/scsi/libfc/fc_lport.c > @@ -311,7 +311,7 @@ struct fc_host_statistics *fc_get_host_stats(struct Scsi_Host *shost) > fc_stats = &lport->host_stats; > memset(fc_stats, 0, sizeof(struct fc_host_statistics)); > > - fc_stats->seconds_since_last_reset = (lport->boot_time - jiffies) / HZ; > + fc_stats->seconds_since_last_reset = (jiffies - lport->boot_time) / HZ; > > for_each_possible_cpu(cpu) { > struct fc_stats *stats; > > This works correctly across jiffies overflows, as long as there is at least one > reset for every jiffies overflow (49 days or more). If we can have longer times > between resets, then we could either use get_jiffies_64() or ktime_get_seconds(). Yes I was going to resend this today, but I'm trapped in the s390 pit... Johannes -- Johannes Thumshirn Storage jthumshirn@suse.de +49 911 74053 689 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg) Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850