linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1271675100.1674.818.camel@laptop>
Date:	Mon, 19 Apr 2010 13:05:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Glauber Costa <glommer@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On Mon, 2010-04-19 at 13:50 +0300, Avi Kivity wrote:
> On 04/19/2010 01:39 PM, Peter Zijlstra wrote:
> > On Fri, 2010-04-16 at 13:36 -0700, Jeremy Fitzhardinge wrote:
> >    
> >>> +     do {
> >>> +             last = last_value;
> >>>
> >>>        
> >> Does this need a barrier() to prevent the compiler from re-reading
> >> last_value for the subsequent lines?  Otherwise "(ret<  last)" and
> >> "return last" could execute with different values for "last".
>     
> > ACCESS_ONCE() is your friend.
> >    
> 
> I think it's implied with atomic64_read().

Yes it would be. I was merely trying to point out that

  last = ACCESS_ONCE(last_value);

Is a narrower way of writing:

  last = last_value;
  barrier();

In that it need not clobber all memory locations and makes it instantly
clear what we want the barrier for.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/