linux-kernel - Re: [PATCH 04/12] misc: xilinx_sdfec: Add open, close and ioctl

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Date:   Tue, 19 Mar 2019 16:36:08 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Dragan Cvetic <draganc@...inx.com>
Cc:     gregkh <gregkh@...uxfoundation.org>,
        Michal Simek <michals@...inx.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Derek Kiernan <dkiernan@...inx.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/12] misc: xilinx_sdfec: Add open, close and ioctl

On Tue, Mar 19, 2019 at 3:59 PM Dragan Cvetic <draganc@...inx.com> wrote:
> >
> > > +       /* Only one open per device at a time */
> > > +       if (!atomic_dec_and_test(&xsdfec->open_count)) {
> > > +               atomic_inc(&xsdfec->open_count);
> > > +               return -EBUSY;
> > > +       }
> >
> > What is that limitation for? Is it worse to open it twice than
> > to dup() or fork()?
> >
> The device can be opened only once.

What I mean here is that preventing the double open() is
a fairly weak protection: it means you cannot have multiple
'struct file' pointers attached to the same inode, but you
can still have the same 'struct file' being available to
multiple processes.

         Arnd