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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 4 Nov 2020 10:38:57 -0800
Message-ID: <CAHk-=wikhGExmprXgaW+MVXG1zsGpztBbVwOb23vetk41EtTBQ@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork
To:     "Ahmed S. Darwish" <a.darwish@linutronix.de>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>, Peter Xu <peterx@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Christoph Hellwig <hch@lst.de>,
        Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
        Jann Horn <jannh@google.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@suse.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Siewior <bigeasy@linutronix.de>
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On Tue, Nov 3, 2020 at 7:17 PM Ahmed S. Darwish <a.darwish@linutronix.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