From mboxrd@z Thu Jan  1 00:00:00 1970
From: Thomas Monjalon <thomas.monjalon@6wind.com>
Subject: Re: [PATCH] eal: fix bug in x86 cmpset
Date: Thu, 09 Mar 2017 16:39:55 +0100
Message-ID: <2831675.mzhxhSnzbD@xps13>
References: <1475184293-18298-1-git-send-email-nikhil.rao@intel.com>
 <2295899.BebvH11edl@xps13> <20170210084644.52651be6@xeon-e3>
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Cc: bruce.richardson@intel.com,
 Konstantin Ananyev <konstantin.ananyev@intel.com>, dev@dpdk.org
To: Stephen Hemminger <stephen@networkplumber.org>, "Hunt,
 David" <david.hunt@intel.com>, Nikhil Rao <nikhil.rao@intel.com>
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2017-02-10 08:46, Stephen Hemminger:
> On Fri, 10 Feb 2017 11:53:06 +0100
> Thomas Monjalon <thomas.monjalon@6wind.com> wrote:
> 
> > 2017-02-10 10:39, Hunt, David:
> > > 
> > > 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.  
> > 
> > This operation is used a lot in drivers for link status.
> > 
> > I think we need to clearly explain what was the consequence of this bug.
> 
> 
> A bigger issue is why there are a huge number of copies of the same link code
> in drivers. Definitely should be common code.  Also why is cmpset used here
> when a simple atomic_set would work as well for what was intended.


I'm surprised that there is no progress on this issue.