linux-kernel - Re: [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page

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Message-ID: <CAHk-=wj2jtBhfgpxCkgXcyOPn3YFFWeJ-vHsppocBjdFQ2F6Og@mail.gmail.com>
Date:   Mon, 2 Nov 2020 14:39:49 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        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>,
        John Hubbard <jhubbard@...dia.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 0/2] Add a seqcount between gup_fast and copy_page_range()

On Mon, Nov 2, 2020 at 2:19 PM Ahmed S. Darwish <a.darwish@...utronix.de> wrote:
>
> Disabling preemption for seqcount_t write-side critical sections was
> never a new requirement. It has always been this way, for the reasons
> explained at Documentation/locking/seqlock.rst, "Introduction" section.

Note that that is only true if you spin on the reading side (either of
the two kinds of spinning: (a) spinning to wait for it to become even,
or (b) repeating if they don't match)

Which this code doesn't do, it just fails.

I'm not sure how to perhaps document that.

             Linus