linux-kernel - Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a configurable timer

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Message-ID: <20150612084251.GB9257@pd.tnic>
Date:	Fri, 12 Jun 2015 10:42:51 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Aaron Lu <aaron.lu@...el.com>, Tony Li <tony.li@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Len Brown <lenb@...nel.org>, Ken Xue <ken.xue@....com>,
	Huang Rui <ray.huang@....com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a
 configurable timer

On Tue, Jun 09, 2015 at 09:46:52AM -0700, Andy Lutomirski wrote:
> I don't like this hack.  The compiler is entirely within is rights to
> poke addr's cacheline (i.e. the stack) between the two instructions.
> I'd suggest either making the thing a full cacheline long or using a
> single asm statement.

How about this:

        /*
         * This should be a memory location in a cache line which is
         * unlikely to be touched by other processors.  The actual
         * content is immaterial as it is not actually modified in any way.
         */
        mwait_ptr = &current_thread_info()->flags;

and then

	__monitor(mwait_ptr, 0, 0);

We already do this in mwait_play_dead().

However, am I even correct in assuming that ->flags won't really be
touched as we're doing delay() and nothing pokes into current anyway?

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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