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

Avi Kivity
2010-04-19T14:37:53Z

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.