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: <4BCC6ABA.9030006@redhat.com>
Date:	Mon, 19 Apr 2010 17:37:46 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On 04/19/2010 05:32 PM, Glauber Costa wrote:
>
>> Right, another option is to put the initial read outside of the loop,
>> that way you'll have the best of all cases, a single LOCK'ed op in the
>> loop, and only a single LOCK'ed op for the fast path on sensible
>> architectures ;-)
>>
>>      last = atomic64_read(&last_value);
>>      
> isn't a barrier enough here?
>
>    

No.  On i386, the statement

    last = last_value;

will be split by the compiler into two 32-bit loads.  If a write 
(atomic, using cmpxchg) on another cpu happens between those two loads, 
then the variable last will have a corrupted value.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

--
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/