netdev - RE: [patch 14/30] drm/i915/pmu: Replace open coded kstat

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6dd2eb7de7ad4a5893f057b90662718f@AcuMS.aculab.com>
Date:   Fri, 11 Dec 2020 22:06:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Marc Zyngier <maz@...nel.org>,
        "Jani Nikula" <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        afzal mohammed <afzal.mohd.ma@...il.com>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Wambui Karuga <wambui.karugax@...il.com>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Lee Jones <lee.jones@...aro.org>, Jon Mason <jdmason@...zu.us>,
        Dave Jiang <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>,
        "linux-ntb@...glegroups.com" <linux-ntb@...glegroups.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Michal Simek" <michal.simek@...inx.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Karthikeyan Mitran <m.karthikeyan@...iveil.co.in>,
        Hou Zhiqiang <Zhiqiang.Hou@....com>,
        "Tariq Toukan" <tariqt@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        "Leon Romanovsky" <leon@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: RE: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy

From: Thomas Gleixner
> Sent: 11 December 2020 21:11
> 
> On Fri, Dec 11 2020 at 14:19, David Laight wrote:
> > From: Thomas Gleixner
> >> You can't catch that. If this really becomes an issue you need a
> >> sequence counter around it.
> >
> > Or just two copies of the high word.
> > Provided the accesses are sequenced:
> > writer:
> > 	load high:low
> > 	add small_value,high:low
> > 	store high
> > 	store low
> > 	store high_copy
> > reader:
> > 	load high_copy
> > 	load low
> > 	load high
> > 	if (high != high_copy)
> > 		low = 0;
> 
> And low = 0 is solving what? You need to loop back and retry until it's
> consistent and then it's nothing else than an open coded sequence count.

If it is a counter or timestamp then the high:0 value happened
some time between when you started trying to read the value and
when you finished trying to read it.

As such it is a perfectly reasonable return value.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)