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

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Message-ID: <20201102231821.GA51715@lx-t490>
Date:   Tue, 3 Nov 2020 00:18:21 +0100
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 02, 2020 at 02:39:49PM -0800, Linus Torvalds wrote:
> 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.
>

Sure, and this is one of the reasons the lockdep non-preemptibility
check is only added to the non-raw variants of the seqcount write APIs.

Presumably, users of the raw_*() part of the API know what they're
doing, and they don't need to read seqlock.rst :)

(I'm in progress of replying to patch #2, which touches a bit on this
 and other points)..

>              Linus

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH