From mboxrd@z Thu Jan  1 00:00:00 1970
From: mark.rutland@arm.com (Mark Rutland)
Date: Wed, 14 Jun 2017 11:42:30 +0100
Subject: [PATCH v8 6/9] drivers: perf: hisi: Add support for Hisilicon
 Djtag driver
In-Reply-To: <20170614100658.GE16190@arm.com>
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>
Message-ID: <20170614104230.GC6085@leverpostej>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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.
IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.