linux-kernel - Re: [PATCH 02/31] fs: add new vfs_poll and file_can

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Date:   Sat, 6 Jan 2018 19:08:53 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Avi Kivity <avi@...lladb.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/31] fs: add new vfs_poll and file_can_poll helpers

On Thu, Jan 04, 2018 at 09:00:14AM +0100, Christoph Hellwig wrote:
> These abstract out calls to the poll method in preparation for changes to
> those methods.

FWIW, I would make vfs_poll()

static inline __poll_t vfs_poll(struct file *file, struct poll_table_struct *pt)
{
	if (unlikely(!file->f_op->poll))
		return DEFAULT_POLLMASK;
	return file->f_op->poll(file, pt);
}

That's safe for any struct file, some of the callers already do just that,
the ones that have vfs_poll() under the check for file_can_poll() will
simply optimize that piece away and the few that rely upon the knowledge
of file_operations they are dealing with (vhost, etc.) can bloody well
cope with the cost of the check.