Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq

Jason Low
2015-05-01T17:40:19Z

On Fri, 2015-05-01 at 08:21 -0700, Paul E. McKenney wrote:
> On Thu, Apr 30, 2015 at 02:13:07PM -0700, Jason Low wrote:
> > On Thu, 2015-04-30 at 14:42 -0400, Waiman Long wrote:
> >
> > > I do have a question of what kind of tearing you are talking about. Do
> > > you mean the tearing due to mm being changed in the middle of the
> > > access? The reason why I don't like this kind of construct is that I am
> > > not sure if
> > > the address translation p->mm->numa_scan_seq is being done once or
> > > twice. I looked at the compiled code and the translation is done only once.
> > >
> > > Anyway, the purpose of READ_ONCE and WRITE_ONCE is not for eliminating
> > > data tearing. They are to make sure that the compiler won't compile away
> > > data access and they are done in the order they appear in the program. I
> > > don't think it is a good idea to associate tearing elimination with
> > > those macros. So I would suggest removing the last sentence in your comment.
> >
> > Yes, I can remove the last sentence in the comment since the main goal
> > was to document that we're access this field without exclusive access.
> >
> > In terms of data tearing, an example would be the write operation gets
> > split into multiple stores (though this is architecture dependent). The
> > idea was that since we're modifying a seq variable without the write
> > lock, we want to remove any forms of optimizations as mentioned above or
> > unpredictable behavior, since READ_ONCE/WRITE_ONCE adds no overhead.
>
> Just to be clear... READ_ONCE() and WRITE_ONCE() do not avoid data tearing
> in cases where the thing read or written is too big for a machine word.

Right, that makes sense. I've updated the comment to instead mention
that it's used to avoid "compiler optimizations".

> If the thing read/written does fit into a machine word and if the location
> read/written is properly aligned, I would be quite surprised if either
> READ_ONCE() or WRITE_ONCE() resulted in any sort of tearing.