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

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Message-ID: <20201103004133.GD52235@lx-t490>
Date:   Tue, 3 Nov 2020 01:41:33 +0100
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.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>,
        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 2/2] mm: prevent gup_fast from racing with COW during
 fork

On Mon, Nov 02, 2020 at 08:25:32PM -0400, Jason Gunthorpe wrote:
> On Tue, Nov 03, 2020 at 01:17:12AM +0100, Ahmed S. Darwish wrote:
>
> > Please stick with the official exported API: raw_write_seqcount_begin().
>
> How did you know this was 'offical exported API' ??
>

All the official exported seqlock.h APIs are marked with verbose
kernel-doc annotations on top. The rest are internal...

> > Exactly. '*_seqcount_t_*' is a seqlock.h implementation detail, and it
> > has _zero_ relevance to what is discussed in this thread actually.
>
> Add some leading __'s to them?
>

It's a bit more complicated than just adding some "__" prefixes, due to
the massive compile-time polymorphism done through _Generic().

The '*_seqcount_t_*' format was the best we could come up with to
distinguish (again, for internal seqlock.h code) between macros taking
all seqcount_LOCKNAME_t types, and macros/functions taking only plain
seqcount_t.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH