linux-kernel - Re: [PATCH v6] x86/hpet: Reduce HPET counter read contention

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Message-ID: <57BF3DE6.1090005@intel.com>
Date:   Thu, 25 Aug 2016 11:50:14 -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 v6] x86/hpet: Reduce HPET counter read contention

On 08/12/2016 05:59 PM, Waiman Long wrote:
> + * The lock and the hpet value are stored together and can be read in a
> + * single atomic 64-bit read. It is explicitly assumed that arch_spinlock_t
> + * is 32 bits in size.

This requirement forces us to give up all of the goodness of lockdep.
Is this strictly a performance optimization or is there some function
requirement behind it as well?