linux-kernel - Re: [PATCH 2/3] driver core: Use rwsem for kill

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Date:   Fri, 31 Jul 2020 08:45:05 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH 2/3] driver core: Use rwsem for kill_device()
 serialization

On Thu, Jul 30, 2020 at 11:56:10AM +0200, Lukas Wunner wrote:
> On Thu, Jul 30, 2020 at 08:53:26AM +0200, Greg Kroah-Hartman wrote:
> > On Wed, Jul 08, 2020 at 03:27:02PM +0200, Lukas Wunner wrote:
> > > kill_device() is currently serialized with driver probing by way of the
> > > device_lock().  We're about to serialize it with device_add() as well
> > > to prevent addition of children below a device which is going away.
> > 
> > Why?  Who does this?  Shouldn't the bus that is trying to do this know
> > this is happening?
> 
> AFAICS, at least spi and i2c are affected.
> 
> I first thought that pci is affected as well but it seems the global
> pci_lock_rescan_remove() performs the required serialization.
> 
> I've yet to take a closer look at acpi and usb.  Any bus which
> creates a device hierarchy with dynamic addition & removal needs
> to make sure no new children are added after removal of the parent
> has begun.
> 
> 
> > So, why are you pushing this down into the driver core, can't this be
> > done in whatever crazy bus wants to do this, like is done here?
> 
> I guess it can.  Let me try to perform the locking at the bus level then.

I thought the bus code itself had this type of serialization already...