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

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Date:   Wed, 14 Jun 2017 11:42:30 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     John Garry <john.garry@...wei.com>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        anurup.m@...wei.com, tanxiaojun@...wei.com, xuwei5@...ilicon.com,
        sanil.kumar@...ilicon.com, gabriele.paoloni@...wei.com,
        shiju.jose@...wei.com, huangdaode@...ilicon.com,
        linuxarm@...wei.com, dikshit.n@...wei.com, shyju.pv@...wei.com,
        anurupvasu@...il.com
Subject: Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon
 Djtag driver

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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