Re: BUG: maple_tree: KCSAN: data-race in mas_topiary_replace / mtree_range_walk [EXPERIMENTAL PATCH]

[Mirsad Todorovac]

On 10/2/23 18:43, Liam R. Howlett wrote:
> * Mirsad Todorovac <mirsad.todorovac@xxxxxxxxxxxx> [231001 17:09]:
>
> ...
>
> > They say that one patch speaks more than a thousand words.
> >
> > I am just running this little patch that actually silences all of the KCSAN warnings.
> >
> > I cannot tell if these reported data races are the actual bugs, but it is possible that
> > the Ubuntu 22.04 gcc is doing some funny stuff when optimising. In Prof. McKenney's
> > book I've read about the load-tearing and store-tearing. AFAICS, memory barriers should
> > prevent load/store reordering, but not the compiler optimisations.
> >
> > Please find two versions of the patch attached.
> >
> > While mas->index and pivots[offset] in maple_range_walk can change concurrently,
> > I am not smart enough to see whether you expect that in your algorithm or is it a potential
> > bug triggered by GCC optimisations and aggressive Ryzen 9 7950X parallelism.
>
> None of this is necessary, for sure.

Thanks for your feedback. If KCSAN is giving false positives, there is quite a lot of them.

I tend to believe on the safe side of the Prof. Paul McKenney, but it is your code.

> I will have to look at this when I have more time to investigate.  This
> will likely not be soon, however.

I see. I also have some tough stuff at my day job this month. :-P

Have a nice day.

Best regards,
Mirsad

> Thanks,
> Liam