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Date:   Thu, 31 Aug 2023 00:40:45 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Bernd Schubert <bschubert@ddn.com>
Cc:     linux-fsdevel@vger.kernel.org, bernd.schubert@fastmail.fm,
        miklos@szeredi.hu, dsingh@ddn.com,
        Josef Bacik <josef@toxicpanda.com>,
        linux-btrfs@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH 0/2] Use exclusive lock for file_remove_privs
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On Wed, Aug 30, 2023 at 08:15:17PM +0200, Bernd Schubert wrote:
> While adding shared direct IO write locks to fuse Miklos noticed
> that file_remove_privs() needs an exclusive lock. I then
> noticed that btrfs actually has the same issue as I had in my patch,
> it was calling into that function with a shared lock.
> This series adds a new exported function file_needs_remove_privs(),
> which used by the follow up btrfs patch and will be used by the
> DIO code path in fuse as well. If that function returns any mask
> the shared lock needs to be dropped and replaced by the exclusive
> variant.

FYI, xfs and ext4 use a simple IS_NOSEC check for this.  That has
a lot more false positives, but also is a much faster check in the
fast path.   It might be worth benchmarking which way to go, but
we should be consistent across file systems for the behavior.