Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

Peter Zijlstra
2016-05-04T13:53:42Z

On Wed, May 04, 2016 at 02:39:40PM +0200, Petr Mladek wrote:
> > + * This barrier also ensures that if another CPU goes through the
> > + * syscall barrier, sees the TIF_PATCH_PENDING writes in
> > + * klp_start_transition(), and calls klp_patch_task(), it also sees the
> > + * above write to the target state. Otherwise it can put the task in
> > + * the wrong universe.
> > + */
>
> By other words, it makes sure that klp_patch_task() will assign the
> right patch_state. Where klp_patch_task() could not be called
> before we set TIF_PATCH_PENDING in klp_start_transition().
>
> > + smp_wmb();
> > +}

So I've not read the patch; but ending a function with an smp_wmb()
feels wrong.

A wmb orders two stores, and I feel both stores should be well visible
in the same function.