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

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Message-ID: <20170907212245.GA19307@jade>
Date:   Thu, 7 Sep 2017 18:22:45 -0300
From:   Gustavo Padovan <gustavo@...ovan.org>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     linux-media@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        linux-fsdevel@...r.kernel.org,
        Riley Andrews <riandrews@...roid.com>
Subject: Re: [PATCH v3 14/15] fs/files: export close_fd() symbol

2017-09-07 Al Viro <viro@...IV.linux.org.uk>:

> On Thu, Sep 07, 2017 at 03:42:25PM -0300, 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.
> 
> NAK.  As soon as the reference is in descriptor table, you *can't* do anything
> to it.  This "sharing" part is complete BS - being _told_ that descriptor is
> there does not matter at all.  That descriptor might be hit with dup2() as
> soon as fd_install() has happened.  Or be closed, or any number of other things.
> 
> You can not take it back.  Once fd_install() is done, it's fucking done, period.
> If V4L2 requires removing it from descriptor table, it's a shitty API and needs
> to be fixed.

Sorry for my lack of knowledge here and thank you for the explanation,
things are a lot clear to me. For some reasons I were trying to delay
the sharing of the fd to a event later. I can delay the install of it
but that my require __fd_install() to be available and exportedi as it
may happen in a thread, but I believe you wouldn't be okay with that either,
is that so?

Gustavo