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

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Message-ID: <CAHk-=wgB8nyOQufpn0o6a5BpJCJPnXvH+kRxApujhsgG+7qAwQ@mail.gmail.com>
Date:   Tue, 3 Nov 2020 09:40:22 -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 Mon, Nov 2, 2020 at 10:52 PM Ahmed S. Darwish
<a.darwish@...utronix.de> wrote:
>
> The problem is, I've already documented seqlock.h to death.... There are
> more comments than code in there, and there is "seqlock.rst" under
> Documentation/ to further describe the big picture.

Well, honestly, I think the correct thing to do is to get rid of the
*_seqcount_t_*() functions entirely.

They add nothing but confusion, and they are entirely misnamed. That's
not the pattern we use for "internal use only" functions, and they are
*very* confusing.

They have other issues too: like raw_write_seqcount_end() not being
usable on its own when preemptibility isn't an issue like here. You
basically _have_ to use raw_write_seqcount_t_end(), because otherwise
it tries to re-enable preemption that was never there.

                   Linus