linux-kernel - Re: [RFC PATCH] phylib: mdio: handle register/unregister/register sequence

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Message-ID: <20130822152757.GA23163@kroah.com>
Date:	Thu, 22 Aug 2013 08:27:57 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Boris BREZILLON <b.brezillon@...rkiz.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Mark Brown <broonie@...nel.org>,
	Nick Bowler <nbowler@...iptictech.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] phylib: mdio: handle register/unregister/register
 sequence

On Thu, Aug 22, 2013 at 02:33:56PM +0200, Boris BREZILLON wrote:
> Hello,
> 
> This patch is a proposal to support the register/unregister/register
> sequence on a given mdio bus.
> 
> I use the register/unregister/register sequence to add a fallback when the
> of_mdiobus_register (this function calls mdiobus_register with phy_mask
> set to ~0) does not register any phy device (because the device tree does
> not define any phy).
> In this case I call mdiobus_unregister and then call mdiobus_register with
> a phy_mask set to 0 to trigger a full mdio bus scan.
> 
> I'm not sure this is the right way to do it (this is why I added RFC in the
> subject).
> 
> Could someone help me figuring out what I should use to implement my fallback ?
> 
> 1) use the register/unregister/register sequence

That will not work.  Well, you might think it would work, but then
things randomly start breaking later on.  Try it with the
KOBJECT_DELAYED_DESTROY build option in linux-next, and watch things go
"boom" :)

The rule is, you should never register a kobject/struct device that you
have previously unregistered before, as you really don't know if
unregistering has finished or not.

sorry,

greg k-h
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