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

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Date:	Tue, 7 Jan 2014 21:45:58 -0800
From:	David Cohen <david.a.cohen@...ux.intel.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	"Tang, Jianqiang" <jianqiang.tang@...el.com>,
	"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
	"sarah.a.sharp@...ux.intel.com" <sarah.a.sharp@...ux.intel.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/PATCH] usb/xhci: avoid kernel panic on xhci_suspend()

Hi Greg,

On Tue, Jan 07, 2014 at 08:16:20PM -0800, Greg KH wrote:
> 
> A: No.
> Q: Should I include quotations after my reply?
> 
> http://daringfireball.net/2007/07/on_top
> 
> On Wed, Jan 08, 2014 at 03:49:07AM +0000, Tang, Jianqiang wrote:
> > Hi,
> >  1) I met this issue one time just boot up our Linux Platform(Kernel3.10) with XHCI driver, then kernel panic happen.
> > 
> >    And this issue reported once by other internal team.
> > 
> >    Nothing special of reproduce step and do not need special Hardware I think.
> > 
> >    Just random issue which will happen when meet the timing condition.
> > 
> >  2) This issue is introduced by this patch:
> > 
> >      https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=596d789a211d134dc5f94d1e5957248c204ef850
> > 
> >    which set all hub autosuspend delay to 0.
> 
> That patch was released in a kernel almost a full year ago, yet we have
> never had a report of this happening before, so are you sure this patch
> is the root cause?

This bug happened in a platform with 1 usb3 host controller + 1 usb3 OTG
controller (Jianqiang, please correct me if I'm wrong).
How common is this configuration out there?

Br, David Cohen

> 
> >    This causes race condition during XHCI driver initialization,
> > 
> >     After USB2 hcd and USB2 root hub finish the initialization, USB2 root hub is functional and auto suspend right now, hence trigger XHCI runtime suspend flow;
> > 
> >     At the same time, XHCI driver continue to initialize the USB3 hcd and assign to xhci->shared_hcd after finish the initialization;
> >     
> >     Since xhci_suspend() use the xhci->shared_hcd, so there is race condition that when XHCI runtime suspend called, xhci->shared_hcd still NULL.
> > 
> >     I think this patch is a fix solution since before XHCI finish the whole initialization, USB2 root hub triggered runtime suspend is mean less and do not need to handle.
> 
> With this patch applied, does the crash go away?
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