linux-kernel - Re: [PATCH v5] x86/hpet: Reduce HPET counter read contention

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Date:	Fri, 12 Aug 2016 14:44:10 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Waiman Long <Waiman.Long@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Borislav Petkov <bp@...e.de>,
	Andy Lutomirski <luto@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Randy Wright <rwright@....com>
Subject: Re: [PATCH v5] x86/hpet: Reduce HPET counter read contention

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.