linux-kernel - RE: [PATCH] usb: core: add debugobjects support for urb object

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Date:	Thu, 26 May 2016 02:15:47 +0000
From:	"Du, Changbin" <changbin.du@...el.com>
To:	Greg KH <gregkh@...uxfoundation.org>,
	Alan Stern <stern@...land.harvard.edu>
CC:	"mathias.nyman@...ux.intel.com" <mathias.nyman@...ux.intel.com>,
	"stefan.koch10@...il.com" <stefan.koch10@...il.com>,
	"hkallweit1@...il.com" <hkallweit1@...il.com>,
	"sergei.shtylyov@...entembedded.com" 
	<sergei.shtylyov@...entembedded.com>,
	"jonas.hesselmann@...mail.de" <jonas.hesselmann@...mail.de>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"aryabinin@...tuozzo.com" <aryabinin@...tuozzo.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"dave@...olabs.net" <dave@...olabs.net>,
	"nikolay@...ulusnetworks.com" <nikolay@...ulusnetworks.com>,
	"dvyukov@...gle.com" <dvyukov@...gle.com>,
	"adrien+dev@...ischi.me" <adrien+dev@...ischi.me>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: core: add debugobjects support for urb object

> On Tue, May 24, 2016 at 03:53:53PM +0800, changbin.du@...el.com wrote:
> > From: "Du, Changbin" <changbin.du@...el.com>
> >
> > Add debugobject support to track the life time of struct urb.
> > This feature help us detect violation of urb operations by
> > generating a warning message from debugobject core. And we fix
> > the possible issues at runtime to avoid oops if we can.
> >
> > I have done some tests with some class drivers, no violation
> > found in them which is good. Expect this feature can be used
> > for debugging future problems.
> >
> > Signed-off-by: Du, Changbin <changbin.du@...el.com>
> 
> I agree with Alan, what use is this code?  Existing drivers all work
> properly because the reference counting of urbs is already present, why
> add duplicate counters?  That actually leads to bugs.  I don't see the
> need for this code, please explain it better if you wish for it to be
> accepted.  What has it found/fixed that we have not found yet?  What
> does it allow you to do that you can't do today with the existing code?
> 
> thanks,
> 
> greg k-h
>
I agree with you two now after checking all urb code. I am sorry to disturb you.
I did have an object lifetime issue, but it is of usb_request. I expect the
cause is usb_request is freed but still pending in udc core. Then I cannot
reproduce it, I even cannot know which function driver the usb_request came
from. I originally want to use debugojects to debug that issue, then I though
this can also help urb debugging.  Obviously I am wrong. As you said reference
counting of urbs is already present. :)
I may add debugojects for device mode usb_request farther. Sometimes it helps.

Thank you,
Du, Changbin