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Date:   Wed, 17 Oct 2018 09:19:06 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Daniel Colascione <dancol@google.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        kernel-team@android.com, John Reck <jreck@google.com>,
        John Stultz <john.stultz@linaro.org>,
        Todd Kjos <tkjos@google.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Jeff Layton <jlayton@kernel.org>,
        Khalid Aziz <khalid.aziz@oracle.com>,
        linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Minchan Kim <minchan@google.com>, Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd
Message-ID: <20181017161906.GA5096@infradead.org>
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On Wed, Oct 17, 2018 at 08:44:01AM -0700, Daniel Colascione wrote:
> > Even if no one changes these specific flags we still need a lock due
> > to rmw cycles on the field.  For example fadvise can set or clear
> > FMODE_RANDOM.  It seems to use file->f_lock for synchronization.
> 
> Compare-and-exchange will suffice, right?

Only if all users use the compare and exchange, and right now they
don't.