Date: Wed, 14 Jun 2017 11:42:30 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: John Garry <john.garry@huawei.com>,
        Shaokun Zhang <zhangshaokun@hisilicon.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        anurup.m@huawei.com, tanxiaojun@huawei.com, xuwei5@hisilicon.com,
        sanil.kumar@hisilicon.com, gabriele.paoloni@huawei.com,
        shiju.jose@huawei.com, huangdaode@hisilicon.com, linuxarm@huawei.com,
        dikshit.n@huawei.com, shyju.pv@huawei.com, anurupvasu@gmail.com
Subject: Re: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon
 Djtag driver
Message-ID: <20170614104230.GC6085@leverpostej>
References: <1495457312-237127-1-git-send-email-zhangshaokun@hisilicon.com>
 <20170608163519.GA19643@leverpostej>
 <8666a0fa-126d-e4a3-ac4b-7962f5d79942@huawei.com>
 <20170609143050.GM13955@arm.com>
 <0fbf57f0-9ff7-4fd4-07c7-c5e86028a7d2@huawei.com>
 <20170614100658.GE16190@arm.com>
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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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