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

[Mark Rutland]

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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Thanks,
Mark.