On 02/11/2015 03:28 PM, Linus Torvalds wrote:
>
>
> On Feb 11, 2015 3:15 PM, "Jeremy Fitzhardinge" <jer...@goop.org
> <mailto:jer...@goop.org>> wrote:
> >
> > Right now it needs to be a locked operation to prevent read-reordering.
> > x86 memory ordering rules state that all writes are seen in a globally
> > consistent order, and are globally ordered wrt reads *on the same
> > addresses*, but reads to different addresses can be reordered wrt to
> writes.
>
> The modern x86 rules are actually much tighter than that.
>
> Every store is a release, and every load is an acquire. So a
> non-atomic store is actually a perfectly fine unlock. All preceding
> stores will be seen by other cpu's before the unlock, and while reads
> can pass stores, they only pass *earlier* stores.
>
Right, so in this particular instance, the read of the SLOWPATH flag
*can't* pass the previous unlock store, hence the need for an atomic
unlock or some other mechanism to prevent the read from being reordered.
J
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