From: Dave Hansen <dave.hansen@intel.com>
To: Waiman Long <Waiman.Long@hpe.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.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>
Subject: Re: [PATCH v5] x86/hpet: Reduce HPET counter read contention
Date: Fri, 12 Aug 2016 14:44:10 -0700 [thread overview]
Message-ID: <57AE432A.9010201@intel.com> (raw)
In-Reply-To: <1471037130-60583-1-git-send-email-Waiman.Long@hpe.com>
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.
next prev parent reply other threads:[~2016-08-12 21:44 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-12 21:25 [PATCH v5] x86/hpet: Reduce HPET counter read contention Waiman Long
2016-08-12 21:38 ` Dave Hansen
2016-08-12 22:50 ` Waiman Long
2016-08-12 21:44 ` Dave Hansen [this message]
2016-08-12 22:58 ` Waiman Long
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