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Subject: Re: [PATCH v5] x86/hpet: Reduce HPET counter read contention
To: Waiman Long <Waiman.Long@hpe.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>
References: <1471037130-60583-1-git-send-email-Waiman.Long@hpe.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org, Borislav Petkov <bp@suse.de>,
        Andy Lutomirski <luto@kernel.org>, Prarit Bhargava <prarit@redhat.com>,
        Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>, Randy Wright <rwright@hpe.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <57AE432A.9010201@intel.com>
Date: Fri, 12 Aug 2016 14:44:10 -0700
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On 08/12/2016 02:25 PM, Waiman Long wrote:
> +	do {
> +		cpu_relax();
> +		new.lockval = READ_ONCE(hpet.lockval);
> +	} while ((new.value == old.value) && raw_spin_is_locked(&new.lock));

While it gets more far-fetched, this isn't guaranteed to make progress
until the saved HPET value actually changes.  You could have a constant
stream of other CPUs going and doing hpet_readl() (and getting the same
value back from a sloooow HPET).  So each time through this loop, this
processor sees (new.value == old.value), and sees the lock held.