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

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Message-ID: <4e4d222c-ed8b-a40d-0cdc-cf152573645c@kernel.dk>
Date:   Wed, 18 Nov 2020 14:27:08 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Michal Kubecek <mkubecek@...e.cz>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eventfd: convert to ->write_iter()

On 11/18/20 12:59 PM, Michal Kubecek wrote:
> 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.

A better justification for this patch is that users like io_uring can
potentially write non-blocking to the file if ->write_iter() is
supported.

-- 
Jens Axboe