From mboxrd@z Thu Jan  1 00:00:00 1970
From: Waiman Long <waiman.long@hp.com>
Subject: Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation
Date: Wed, 18 Dec 2013 13:51:02 -0500
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>, 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" <linux-arch@vger.kernel.org>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Peter Zijlstra <peterz@infradead.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>, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, George Spelvin <linux@horizon.com>, Tim Chen <tim.c.chen@linux.intel.com>, Aswin Chandramouleeswaran <aswin@hp.com>, Scott J Norton <scott.norton@hp.com>

On 12/17/2013 02:27 PM, Linus Torvalds wrote:
> On Tue, Dec 17, 2013 at 11:21 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com>  wrote:
>> Looks like xadd() is x86-specific, but this is common code.  One
>> approach would be to do xadd() for other arches, another approach
>> would be to make .rw be an atomic_t rather than a u32.  Making it
>> be atomic_t is probably easiest.  (The cmpxchg()s would then need
>> to be atomic_cmpxchg().)
> Note that "xadd()" has different semantics from "atomic_add_return()".
>
> xadd() returns the original value, while atomic_add_return() returns
> the result of the addition.
>
> In this case, we seem to want the xadd() semantics. I guess we can use
> "atomic_add_return(val,&atomic)-val" and just assume that the compiler
> gets it right (with the addition and the subtraction cancelling out).
> Or maybe we should have a "atomic_add_return_original()" with xadd
> semantics?
>
>                 Linus

I will use the atomic_add_return() for the combined count. Actually what 
the code is looking for in the returned value is the state of the writer 
byte. So it doesn't really matter if _QR_BIAS has or hasn't been added 
to the count.

-Longman