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

[Avi Kivity]

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@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/