Re: [PATCH v3 2/2] mm: prevent gup_fast from racing with COW during fork

[Ahmed S. Darwish]

Hi Jason,

On Fri, Nov 06, 2020 at 11:55:14AM -0400, Jason Gunthorpe wrote:
...
> +     if (gup_flags & FOLL_PIN) {
> +             seq = raw_read_seqcount(&current->mm->write_protect_seq);
> +             if (seq & 1)
> +                     return 0;
> +     }
> +
...
> +     if (gup_flags & FOLL_PIN) {
> +             if (read_seqcount_t_retry(&current->mm->write_protect_seq,
> +                                       seq)) {
> +                     unpin_user_pages(pages, nr_pinned);
> +                     return 0;
> +             }
> +     }

>From seqlock.h:

    /**
     * raw_read_seqcount() - read the raw seqcount_t counter value
     * ...
     * Return: count to be passed to read_seqcount_retry()
     */
    #define raw_read_seqcount(s)

Please avoid using the internal API (read_seqcount_*t*_retry) and just
use read_seqcount_retry() as the documentation suggests.

(I guess you just missed changing that last one... I'm in process of
 changing all these "*_seqcount_t_*" stuff to "do_*" as we talked on the
 v2 thread. Hopefully there will be no more confusion after that.)

Kind regards,

--
Ahmed S. Darwish
Linutronix GmbH