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From: David Laight <David.Laight@ACULAB.COM>
To: 'Thomas Gleixner' <tglx@linutronix.de>,
	Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>
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	"David S. Miller" <davem@davemloft.net>
Subject: RE: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy
Date: Fri, 11 Dec 2020 14:19:05 +0000
Message-ID: <d6cbfa118490459bb0671394f00323fc@AcuMS.aculab.com> (raw)
In-Reply-To: <87y2i4h54i.fsf@nanos.tec.linutronix.de>

From: Thomas Gleixner
> Sent: 11 December 2020 12:58
..
> > After my failed hasty sketch from last night I had a different one which
> > was kind of heuristics based (re-reading the upper dword and retrying if
> > it changed on 32-bit).
> 
> The problem is that there will be two seperate modifications for the low
> and high word. Several ways how the compiler can translate this, but the
> problem is the same for all of them:
> 
> CPU 0                           CPU 1
>         load low
>         load high
>         add  low, 1
>         addc high, 0
>         store low               load high
> --> NMI                         load low
>                                 load high and compare
>         store high
> 
> 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;

The read value is always stale, so it probably doesn't
matter that the value you have is one that is between the
value when you started and that when you finished.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-10 19:25 [patch 00/30] genirq: Treewide hunt for irq descriptor abuse and assorted fixes Thomas Gleixner
2020-12-10 19:25 ` [patch 01/30] genirq: Move irq_has_action() into core code Thomas Gleixner
2020-12-10 19:25 ` [patch 02/30] genirq: Move status flag checks to core Thomas Gleixner
2020-12-27 19:20   ` Guenter Roeck
2021-01-11 10:14     ` Thomas Gleixner
2020-12-10 19:25 ` [patch 03/30] genirq: Move irq_set_lockdep_class() " Thomas Gleixner
2020-12-11 17:53   ` Andy Shevchenko
2020-12-11 21:08     ` Thomas Gleixner
2020-12-11 22:07       ` Thomas Gleixner
2020-12-12 13:22         ` Andy Shevchenko
2020-12-10 19:25 ` [patch 04/30] genirq: Provide irq_get_effective_affinity() Thomas Gleixner
2020-12-10 19:25 ` [patch 05/30] genirq: Annotate irq stats data races Thomas Gleixner
2020-12-10 19:25 ` [patch 06/30] parisc/irq: Simplify irq count output for /proc/interrupts Thomas Gleixner
2020-12-10 19:25 ` [patch 07/30] genirq: Make kstat_irqs() static Thomas Gleixner
2020-12-10 19:25 ` [patch 08/30] genirq: Provide kstat_irqdesc_cpu() Thomas Gleixner
2020-12-10 19:25 ` [patch 09/30] ARM: smp: Use irq_desc_kstat_cpu() in show_ipi_list() Thomas Gleixner
2020-12-11 18:08   ` Marc Zyngier
2020-12-10 19:25 ` [patch 10/30] arm64/smp: Use irq_desc_kstat_cpu() in arch_show_interrupts() Thomas Gleixner
2020-12-11 18:08   ` Marc Zyngier
2020-12-10 19:25 ` [patch 11/30] parisc/irq: Use irq_desc_kstat_cpu() in show_interrupts() Thomas Gleixner
2020-12-10 19:25 ` [patch 12/30] s390/irq: Use irq_desc_kstat_cpu() in show_msi_interrupt() Thomas Gleixner
2020-12-10 20:31   ` Heiko Carstens
2020-12-10 19:25 ` [patch 13/30] drm/i915/lpe_audio: Remove pointless irq_to_desc() usage Thomas Gleixner
2020-12-10 19:48   ` [Intel-gfx] " Ville Syrjälä
2020-12-11  9:51     ` Jani Nikula
2020-12-10 19:25 ` [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy Thomas Gleixner
2020-12-11  9:54   ` Jani Nikula
2020-12-11 10:13   ` Tvrtko Ursulin
2020-12-11 12:57     ` Thomas Gleixner
2020-12-11 14:19       ` David Laight [this message]
2020-12-11 21:10         ` Thomas Gleixner
2020-12-11 22:06           ` David Laight
2020-12-10 19:25 ` [patch 15/30] pinctrl: nomadik: Use irq_has_action() Thomas Gleixner
2020-12-12  0:45   ` Linus Walleij
2020-12-10 19:25 ` [patch 16/30] mfd: ab8500-debugfs: Remove the racy fiddling with irq_desc Thomas Gleixner
2020-12-11  8:22   ` Linus Walleij
2020-12-11 10:04   ` Lee Jones
2020-12-11 18:12   ` Andy Shevchenko
2020-12-10 19:25 ` [patch 17/30] NTB/msi: Use irq_has_action() Thomas Gleixner
2020-12-10 20:33   ` Logan Gunthorpe
2020-12-10 19:25 ` [patch 18/30] PCI: xilinx-nwl: Use irq_data_get_irq_chip_data() Thomas Gleixner
2020-12-10 22:56   ` Rob Herring
2020-12-10 19:25 ` [patch 19/30] PCI: mobiveil: " Thomas Gleixner
2020-12-10 22:56   ` Rob Herring
2020-12-10 19:25 ` [patch 20/30] net/mlx4: Replace irq_to_desc() abuse Thomas Gleixner
2020-12-13 11:24   ` Tariq Toukan
2020-12-10 19:25 ` [patch 21/30] net/mlx4: Use effective interrupt affinity Thomas Gleixner
2020-12-13 11:31   ` Tariq Toukan
2020-12-10 19:25 ` [patch 22/30] net/mlx5: Replace irq_to_desc() abuse Thomas Gleixner
2020-12-13 11:34   ` Tariq Toukan
2020-12-14 21:13   ` Saeed Mahameed
2020-12-10 19:25 ` [patch 23/30] net/mlx5: Use effective interrupt affinity Thomas Gleixner
2020-12-13 11:35   ` Tariq Toukan
2020-12-14 20:58   ` Saeed Mahameed
2020-12-10 19:26 ` [patch 24/30] xen/events: Remove unused bind_evtchn_to_irq_lateeoi() Thomas Gleixner
2020-12-10 23:19   ` boris.ostrovsky
2020-12-11  0:04     ` Thomas Gleixner
2020-12-10 19:26 ` [patch 25/30] xen/events: Remove disfunct affinity spreading Thomas Gleixner
2020-12-10 19:26 ` [patch 26/30] xen/events: Use immediate affinity setting Thomas Gleixner
2020-12-10 19:26 ` [patch 27/30] xen/events: Only force affinity mask for percpu interrupts Thomas Gleixner
2020-12-10 23:20   ` boris.ostrovsky
2020-12-11  0:06     ` Thomas Gleixner
2020-12-11  6:17     ` Jürgen Groß
     [not found]       ` <874kksiras.fsf@nanos.tec.linutronix.de>
2020-12-11 10:23         ` Jürgen Groß
2020-12-11 12:10     ` Jürgen Groß
2020-12-11 12:37       ` Thomas Gleixner
2020-12-11 14:29         ` boris.ostrovsky
2020-12-11 21:27           ` Thomas Gleixner
2020-12-11 22:21             ` Andrew Cooper
2020-12-11 22:56               ` Thomas Gleixner
2020-12-10 19:26 ` [patch 28/30] xen/events: Reduce irq_info::spurious_cnt storage size Thomas Gleixner
2020-12-10 19:26 ` [patch 29/30] xen/events: Implement irq distribution Thomas Gleixner
2020-12-10 19:26 ` [patch 30/30] genirq: Remove export of irq_to_desc() Thomas Gleixner

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