linux-kernel - Re: [PATCH] eventfd: convert to ->write

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Date:   Wed, 18 Nov 2020 20:59:36 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eventfd: convert to ->write_iter()

On Wed, Nov 18, 2020 at 03:18:06PM +0000, Christoph Hellwig wrote:
> On Wed, Nov 18, 2020 at 10:19:17AM +0100, Michal Kubecek wrote:
> > While eventfd ->read() callback was replaced by ->read_iter() recently,
> > it still provides ->write() for writes. Since commit 4d03e3cc5982 ("fs:
> > don't allow kernel reads and writes without iter ops"), this prevents
> > kernel_write() to be used for eventfd and with set_fs() removal,
> > ->write() cannot be easily called directly with a kernel buffer.
> > 
> > According to eventfd(2), eventfd descriptors are supposed to be (also)
> > used by kernel to notify userspace applications of events which now
> > requires ->write_iter() op to be available (and ->write() not to be).
> > Therefore convert eventfd_write() to ->write_iter() semantics. This
> > patch also cleans up the code in a similar way as commit 12aceb89b0bc
> > ("eventfd: convert to f_op->read_iter()") did in read_iter().
> 
> A far as I can tell we don't have an in-tree user that writes to an
> eventfd.  We can merge something like this once there is a user.

As far as I can say, we don't have an in-tree user that reads from
sysctl. But you not only did not object to commit 4bd6a7353ee1 ("sysctl:
Convert to iter interfaces") which adds ->read_iter() for sysctl, that
commit even bears your Signed-off-by. There may be other examples like
that.

Michal Kubecek