linux-kernel - Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork

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Date:   Wed, 4 Nov 2020 10:38:57 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork

On Tue, Nov 3, 2020 at 7:17 PM Ahmed S. Darwish <a.darwish@...utronix.de> wrote:
>
> Nonetheless, as you mentioned in the later (dropped) part of your
> message, I think do_ is better than __do_, so the final result will be:
>
>   do___read_seqcount_retry()
>   do_read_seqcount_retry()
>   do_raw_write_seqcount_begin()
>   do_raw_write_seqcount_end()
>   do_write_seqcount_begin()
>   ...
>
> and so on.

Looks reasonable to me.

And can you add a few comments to the magic type macros, so that it's
a lot more obvious what the end result was. I clearly wasn't able to
follow all the _Generic() cases from the seqcount_t to the final end
result. It's a really odd combination of subtle _GENERIC() macro and
token pasting to get from zeqcount_t to "false" in
__seqcount_lock_preemptible().

I can see it when I really look, but when looking at the actual use,
it's very non-obvious indeed.

                 Linus