linux-kernel - Re: [PATCH v3 14/15] fs/files: export close

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Date:   Thu, 07 Sep 2017 19:18:51 -0300
From:   Gustavo Padovan <gustavo.padovan@...labora.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Gustavo Padovan <gustavo@...ovan.org>,
        linux-media@...r.kernel.org
Cc:     Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        Riley Andrews <riandrews@...roid.com>
Subject: Re: [PATCH v3 14/15] fs/files: export close_fd() symbol

On Fri, 2017-09-08 at 00:09 +0200, Hans Verkuil wrote:
> On 09/07/2017 08:42 PM, Gustavo Padovan wrote:
> > From: Gustavo Padovan <gustavo.padovan@...labora.com>
> > 
> > Rename __close_fd() to close_fd() and export it to be able close
> > files
> > in modules using file descriptors.
> > 
> > The usecase that motivates this change happens in V4L2 where we
> > send
> > events to userspace with a fd that has file installed in it. But if
> > for
> > some reason we have to cancel the video stream we need to close the
> > files
> > that haven't been shared with userspace yet. Thus the export of
> > close_fd() becomes necessary.
> > 
> > fd_install() happens when we call an ioctl to queue a buffer, but
> > we only
> > share the fd with userspace later, and that may happen in a kernel
> > thread
> > instead.
> 
> This isn't the way to do this.
> 
> You should only create the out fence file descriptor when userspace
> dequeues
> (i.e. calls VIDIOC_DQEVENT) the BUF_QUEUED event. That's when you
> give it to
> userspace and at that moment closing the fd is the responsibility of
> userspace.
> There is no point creating it earlier anyway since userspace can't
> get to it
> until it dequeues the event.
> 
> It does mean some more work in the V4L2 core since you need to hook
> into the
> DQEVENT code in order to do this.

Right, that makes a lot more sense. I'll change the implementation so
it can reflecting that. Thanks.

Gustavo