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

Paul E. McKenney
2015-05-01T15:22:06Z

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.
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.

Thanx, Paul