netdev - Re: [PATCH 13/36] fs: introduce new ->get_poll_head and ->poll

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Date:   Tue, 20 Mar 2018 16:39:00 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Christoph Hellwig <hch@....de>, viro@...iv.linux.org.uk,
        Avi Kivity <avi@...lladb.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/36] fs: introduce new ->get_poll_head and
        ->poll_mask methods

On Mon, Mar 19, 2018 at 08:29:38PM -0700, Darrick J. Wong wrote:
> On Mon, Mar 05, 2018 at 01:27:20PM -0800, Christoph Hellwig wrote:
> > ->get_poll_head returns the waitqueue that the poll operation is going
> > to sleep on.  Note that this means we can only use a single waitqueue
> > for the poll, unlike some current drivers that use two waitqueues for
> > different events.  But now that we have keyed wakeups and heavily use
> > those for poll there aren't that many good reason left to keep the
> > multiple waitqueues, and if there are any ->poll is still around, the
> > driver just won't support aio poll.
> > 
> > Signed-off-by: Christoph Hellwig <hch@....de>
> 
> I've been wondering, how does a regular filesystem connect with this?

In general, it doesn't.  In Unix regular files aren't pollable.

> Also, does anything implement get_poll_head?  It looks to me like an aio
> poll provider has to provide both...

Yes.  Everyone who implements ->poll_mask also needs to implement
get_poll_head.  For sockets we just happen to be able to use a generic
implementation in net/socket.c for most of them.