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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
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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.