From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <david.hunt@intel.com>
Received: from mga05.intel.com (mga05.intel.com [192.55.52.43])
 by dpdk.org (Postfix) with ESMTP id 922883B5
 for <dev@dpdk.org>; Fri, 10 Feb 2017 11:39:21 +0100 (CET)
Received: from orsmga004.jf.intel.com ([10.7.209.38])
 by fmsmga105.fm.intel.com with ESMTP; 10 Feb 2017 02:39:20 -0800
X-ExtLoop1: 1
X-IronPort-AV: E=Sophos;i="5.35,140,1484035200"; d="scan'208";a="57236091"
Received: from dhunt5-mobl.ger.corp.intel.com (HELO [10.237.221.69])
 ([10.237.221.69])
 by orsmga004.jf.intel.com with ESMTP; 10 Feb 2017 02:39:19 -0800
To: Thomas Monjalon <thomas.monjalon@6wind.com>,
 Nikhil Rao <nikhil.rao@intel.com>, bruce.richardson@intel.com,
 Konstantin Ananyev <konstantin.ananyev@intel.com>
References: <1475184293-18298-1-git-send-email-nikhil.rao@intel.com>
 <4122748.D2hJ4kS1ho@xps13> <2431986.cxPJYuksjQ@xps13>
 <1919498.PKpEFfz702@xps13>
Cc: dev@dpdk.org
From: "Hunt, David" <david.hunt@intel.com>
Message-ID: <b04df489-8d3e-bc48-a69e-0a6b3bea0669@intel.com>
Date: Fri, 10 Feb 2017 10:39:18 +0000
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:45.0) Gecko/20100101
 Thunderbird/45.7.1
MIME-Version: 1.0
In-Reply-To: <1919498.PKpEFfz702@xps13>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Re: [dpdk-dev] [PATCH] eal: fix bug in x86 cmpset
X-BeenThere: dev@dpdk.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: DPDK patches and discussions <dev.dpdk.org>
List-Unsubscribe: <http://dpdk.org/ml/options/dev>,
 <mailto:dev-request@dpdk.org?subject=unsubscribe>
List-Archive: <http://dpdk.org/ml/archives/dev/>
List-Post: <mailto:dev@dpdk.org>
List-Help: <mailto:dev-request@dpdk.org?subject=help>
List-Subscribe: <http://dpdk.org/ml/listinfo/dev>,
 <mailto:dev-request@dpdk.org?subject=subscribe>
X-List-Received-Date: Fri, 10 Feb 2017 10:39:22 -0000


On 9/2/2017 4:53 PM, Thomas Monjalon wrote:
> 2016-11-06 22:09, Thomas Monjalon:
>> 2016-09-29 18:34, Thomas Monjalon:
>>> 2016-09-30 02:54, Nikhil Rao:
>>>> The original code used movl instead of xchgl, this caused
>>>> rte_atomic64_cmpset to use ebx as the lower dword of the source
>>>> to cmpxchg8b instead of the lower dword of function argument "src".
>>> Could you please start the explanation with a statement of
>>> what is wrong from an user point of view?
>>> It could help to understand how severe it is.
>> Please, we need a clear explanation of the bug, and an acknowledgement.
> Should we close this bug?

I took a few minutes to look at this, and the issue can easily be 
reproduced with a small snippet of code.
With the 'mov', the lower dword of the result is incorrect. This is 
resolved by using 'xchgl'.

void main()
{
         uint64_t a = 0xff000000ff;

         rte_atomic64_cmpset( &a, 0xff000000ff, 0xfa000000fa);
         printf("0x%lx\n", a);
}

When using 'mov', the result is 0xfa00000000
When using 'xchgl', the result is 0xfa000000fa, as expected.

Rgds,
Dave.