linux-kernel - Re: [RFC/PATCH] usb/xhci: avoid kernel panic on xhci

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Date:	Wed, 8 Jan 2014 10:48:06 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Greg KH <gregkh@...uxfoundation.org>
cc:	David Cohen <david.a.cohen@...ux.intel.com>,
	<sarah.a.sharp@...ux.intel.com>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, jianqian <jianqiang.tang@...el.com>
Subject: Re: [RFC/PATCH] usb/xhci: avoid kernel panic on xhci_suspend()

On Tue, 7 Jan 2014, Greg KH wrote:

> On Tue, Jan 07, 2014 at 05:44:26PM -0800, David Cohen wrote:
> > From: jianqian <jianqiang.tang@...el.com>
> > 
> > There is a possible kernel panic faced on xhci_suspend().
> > Due to kernel modified the hub autosupend_delay to 0s, after usb1 root
> > hub finishes initialization, it will trigger runtime_suspend and then
> > it will trigger xhci runtime suspend. But at that time, if
> > xhci->shared_hcd is still doing initialization, it is possible to face
> > null pointer kernel panic in xhci_suspend() function.
> > 
> > This patch checks if xhci->shared_hcd is null to avoid panic.
> 
> That sounds like this is a race that should be fixed properly, not just
> papered over, right?

That was my reaction too.  The best way to solve the problem is to 
prevent the USB-2 root hub from suspending until after the USB-3 root 
hub has been registered.

Alan Stern

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