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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 13 Dec 2018 12:05:36 +0100
Message-ID: <CAJZ5v0g9BGP61CT6SSypo+sUa8BbE+cPuqqzwKfKW=b8YNM6Wg@mail.gmail.com>
Subject: Re: [PATCH] drivers/base: use a worker for sysfs unbind
To:     Daniel Vetter <daniel.vetter@intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        ramalingam.c@intel.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>
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On Thu, Dec 13, 2018 at 11:23 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Thu, Dec 13, 2018 at 10:58 AM Daniel Vetter <daniel@ffwll.ch> wrote:

[cut]

> >
> > - Most of these issues are never visible in normal usage, since normally
> >   driver bind/unbind is done from a kthread or model_load/unload, neither
> >   of which is running in the context of that kernfs mutex kernfs_fop_write
> >   holds. That's why I think the task work is the best solution, since it
> >   changes the locking context of the unbind sysfs to match the locking
> >   context of module unload and hotunplug.
>
> I think that using a task work here makes sense.  There is a drawback,
> which is that the original sysfs write will not wait for the driver to
> actually be released before returning to user space AFAICS, but that
> probably isn't a big deal.
>
> Also please note that the patch changes the code flow slightly,
> because passing a non-NULL parent pointer to
> device_release_driver_internal() potentially has side effects, but
> that should not be a big deal either.
>
> > Unfortunately that trick doesn't work for the bind sysfs file, since that way we can't thread the errno value back to userspace.
>
> Right.  That is unless we wait for the operation to complete and check
> the error left behind by it.  That should be doable, but somewhat
> complicated.

That said I'm not really sure if propagating the error to user space
in this case should be expected.  The interface could be defined as
asynchronous to begin with a separate way for user space to check the
status if necessary.  Changing that now may not be practical, though.