Message-ID: <5303FFC5.5040004@hp.com>
Date: Tue, 18 Feb 2014 19:50:13 -0500
From: Waiman Long <waiman.long@hp.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        linux-arch@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>,
        Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        George Spelvin <linux@horizon.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Daniel J Blueman <daniel@numascale.com>,
        Alexander Fyodorov <halcy@yandex.ru>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>,
        Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
Subject: Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock
 implementation
References: <1392669684-4807-1-git-send-email-Waiman.Long@hp.com> <1392669684-4807-2-git-send-email-Waiman.Long@hp.com> <20140218073951.GZ27965@twins.programming.kicks-ass.net> <5303B6F3.9090001@hp.com> <20140218213400.GS14089@laptop.programming.kicks-ass.net>
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On 02/18/2014 04:34 PM, Peter Zijlstra wrote:
> On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
>> The #ifdef is harder to take away here. The point is that doing a 32-bit
>> exchange may accidentally steal the lock with the additional code to handle
>> that. Doing a 16-bit exchange, on the other hand, will never steal the lock
>> and so don't need the extra handling code. I could construct a function with
>> different return values to handle the different cases if you think it will
>> make the code easier to read.
> Does it really pay to use xchg() with all those fixup cases? Why not
> have a single cmpxchg() loop that does just the exact atomic op you
> want?

The main reason for using xchg instead of cmpxchg is its performance 
impact when the lock is heavily contended. Under those circumstances, a 
task may need to do several tries of read+atomic-RMV before getting it 
right. This may cause a lot of cacheline contention. With xchg, we need 
at most 2 atomic ops. Using cmpxchg() does simplify the code a bit at 
the expense of performance with heavy contention.

-Longman
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