MIME-Version: 1.0
In-Reply-To: <20160303170532.GC1092@dvhart-mobl5.amr.corp.intel.com>
References: <1457019485-26441-1-git-send-email-nasa4836@gmail.com> <20160303170532.GC1092@dvhart-mobl5.amr.corp.intel.com>
From: Jianyu Zhan <nasa4836@gmail.com>
Date: Fri, 4 Mar 2016 09:12:31 +0800
Message-ID: <CAHz2CGXBTmNVn3e7+7c0WHny6=h2ey9eUAydOimyKOzoqJRdFw@mail.gmail.com>
Subject: Re: [PATCH] futex: replace bare barrier() with more lightweight READ_ONCE()
To: Darren Hart <dvhart@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, dave@stgolabs.net,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>, dvhart@linux.intel.com,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Fengguang Wu <fengguang.wu@intel.com>, bigeasy@linutronix.de
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1365
Lines: 37

On Fri, Mar 4, 2016 at 1:05 AM, Darren Hart <dvhart@infradead.org> wrote:
> I thought I provided a corrected comment block.... maybe I didn't. We have been
> working on improving the futex documentation, so we're paying close attention to
> terminology as well as grammar. This one needs a couple minor tweaks. I suggest:
>
> /*
>  * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
>  * optimizing lock_ptr out of the logic below.
>  */
>
> The bit about q->lock_ptr possibly changing is already covered by the large
> comment block below the spin_lock(lock_ptr) call.

The large comment block is explaining the why the retry logic is required.
To achieve this semantic requirement,  the READ_ONCE is needed to prevent
compiler optimizing it by doing double loads.

So I think the comment above should explain this tricky part.

> /* Use READ_ONCE to forbid the compiler from reloading q->lock_ptr  in spin_lock()  */

And as for preventing from optimizing the lock_ptr out of the retry
code block,  I have consult
Paul Mckenney,  he suggests one more  READ_ONCE should be added here:

if (unlikely(lock_ptr != READ_ONCE(q->lock_ptr))) {
<------------------------------
                  spin_unlock(lock_ptr);
                  goto retry;
 }

And I think this are two problem, and should be separated into two patches?


Regards,
Jianyu Zhan