linux-kernel - Re: pagecache locking (was: bcachefs status update) merged)

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Message-ID: <CAHk-=wgz+7O0pdn8Wfxc5EQKNy44FTtf4LAPO1WgCidNjxbWzg@mail.gmail.com>
Date:   Fri, 14 Jun 2019 18:01:07 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: pagecache locking (was: bcachefs status update) merged)

On Thu, Jun 13, 2019 at 5:08 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I do not believe that posix itself actually requires that at all,
> although extended standards may.

So I tried to see if I could find what this perhaps alludes to.

And I suspect it's not in the read/write thing, but the pthreads side
talks about atomicity.

Interesting, but I doubt if that's actually really intentional, since
the non-thread read/write behavior specifically seems to avoid the
whole concurrency issue.

The pthreads atomicity thing seems to be about not splitting up IO and
doing it in chunks when you have m:n threading models, but can be
(mis-)construed to have threads given higher atomicity guarantees than
processes.

               Linus