Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon Djtag driver

From	Mark Rutland <mark.rutland@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon Djtag driver
Date	2017-06-14 13:00 +0200
Message-ID	<tSebM-48n-29@gated-at.bofh.it> (permalink)
References	(2 earlier) <tQsVA-2Cx-15@gated-at.bofh.it> <tQteW-2JK-3@gated-at.bofh.it> <tQtRD-3bI-3@gated-at.bofh.it> <tSdz4-3TR-17@gated-at.bofh.it> <tSebM-48n-31@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Wed, Jun 14, 2017 at 11:42:30AM +0100, Mark Rutland wrote:
> On Wed, Jun 14, 2017 at 11:06:58AM +0100, Will Deacon wrote:
> > Apologies, I misunderstood your algorithm (I thought step (a) was on one CPU
> > and step (b) was on another). Still, I don't understand the need for the
> > timeout. If you instead read back the flag immediately, wouldn't it still
> > work? e.g.
> >
> >
> > lock:
> >   Readl_relaxed flag
> >   if (locked)
> >     goto lock;
> >
> >   Writel_relaxed unique ID to flag
> >   Readl flag
> >   if (locked by somebody else)
> >     goto lock;
> >
> > <critical section>
> >
> > unlock:
> >   Writel unlocked value to flag
> >
> >
> > Given that we're dealing with iomem, I think it will work, but I could be
> > missing something obvious.
> 
> Don't we have the race below where both threads can enter the critical
> section?
> 
>         // flag f initial zero (unlocked)
> 
>         // t1, flag 1                   // t2, flag 2
>         readl(f); // reads 0            l = readl(f); // reads 0
> 
>         <thinks lock is free>           <thinks lock is free>
> 
>         writel(1, f);
>         readl(f); // reads 1
>         <thinks lock owned>
>                                         writel(2, f);
>                                         readl(f) // reads 2
>                                         <thinks lock owned>
> 
>         <crticial section>              <critical section>
> 
> Thanks,
> Mark.

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Please ignore the disclaimer on this mail; my client was mis-configured.

I will ensure I avoid sending bogus disclaimers in future.

Thanks,
Mark.

Thread

Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Mark Rutland <mark.rutland@arm.com> - 2017-06-08 18:40 +0200
  Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver John Garry <john.garry@huawei.com> - 2017-06-09 16:20 +0200
    Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Will Deacon <will.deacon@arm.com> - 2017-06-09 16:40 +0200
      Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver John Garry <john.garry@huawei.com> - 2017-06-09 17:20 +0200
        Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Will Deacon <will.deacon@arm.com> - 2017-06-14 12:20 +0200
          Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-06-14 12:50 +0200
            Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Will Deacon <will.deacon@arm.com> - 2017-06-14 13:10 +0200
          Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Mark Rutland <mark.rutland@arm.com> - 2017-06-14 13:00 +0200
          Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Will Deacon <will.deacon@arm.com> - 2017-06-14 13:10 +0200
            Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver John Garry <john.garry@huawei.com> - 2017-06-14 13:40 +0200
              Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Will Deacon <will.deacon@arm.com> - 2017-06-14 13:50 +0200
                Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver John Garry <john.garry@huawei.com> - 2017-06-14 14:00 +0200
    Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Mark Rutland <mark.rutland@arm.com> - 2017-06-09 17:50 +0200
      Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver John Garry <john.garry@huawei.com> - 2017-06-09 18:10 +0200
        Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Mark Rutland <mark.rutland@arm.com> - 2017-06-09 18:50 +0200
  Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon  Djtag driver Zhangshaokun <zhangshaokun@hisilicon.com> - 2017-06-14 10:20 +0200

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