linux-kernel - Re: kobject: delayed kobject release: help find buggy drivers

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Message-ID: <87zjrt6tz4.fsf@rustcorp.com.au>
Date:	Wed, 04 Sep 2013 14:53:11 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Greg KH <gregkh@...uxfoundation.org>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	rmk+kernel@....linux.org.uk
Subject: Re: kobject: delayed kobject release: help find buggy drivers

Greg KH <gregkh@...uxfoundation.org> writes:
> On Tue, Sep 03, 2013 at 04:44:42PM -0400, Dave Jones wrote:
>> On Tue, Sep 03, 2013 at 06:59:24PM +0000, Linux Kernel wrote:
>>  > Gitweb:     http://git.kernel.org/linus/;a=commit;h=c817a67ecba7c3c2aaa104796d78f160af60920d
>>  > Commit:     c817a67ecba7c3c2aaa104796d78f160af60920d
>>  > Parent:     7c42721fe0c58a848849b43ff558cf2fb86aa35a
>>  > Author:     Russell King <rmk+kernel@....linux.org.uk>
>>  > AuthorDate: Thu Jun 27 15:06:14 2013 +0100
>>  > Committer:  Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>  > CommitDate: Thu Jul 25 15:39:04 2013 -0700
>>  > 
>>  >     kobject: delayed kobject release: help find buggy drivers
>>  >     
>>  >     Implement debugging for kobject release functions.  kobjects are
>>  >     reference counted, so the drop of the last reference to them is not
>>  >     predictable. However, the common case is for the last reference to be
>>  >     the kobject's removal from a subsystem, which results in the release
>>  >     function being immediately called.
>>  >     
>>  >     This can hide subtle bugs, which can occur when another thread holds a
>>  >     reference to the kobject at the same time that a kobject is removed.
>>  >     This results in the release method being delayed.
>>  >     
>>  >     In order to make these kinds of problems more visible, the following
>>  >     patch implements a delayed release; this has the effect that the
>>  >     release function will be out of order with respect to the removal of
>>  >     the kobject in the same manner that it would be if a reference was
>>  >     being held.
>>  >     
>>  >     This provides us with an easy way to allow driver writers to debug
>>  >     their drivers and fix otherwise hidden problems.
>>  >     
>>  >     Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
>>  >     Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> 
>> With this enabled, I get a bunch of scrolling oopses immediately after
>> exiting the bootloader. It happens so early I can't even capture them
>> over usb-serial, or earlyprintk=dbgp.
>> 
>> And for whatever reason, the printk path while oopsing ignores boot_delay parameter,
>> so I can't even use that.
>> 
>> any ideas ?
>
> The first big bug found with this was with the module kobject code, and
> a fix for that should be going in through Rusty's tree to Linus for this
> merge window (right Rusty?)

Yep, just sent pull request.  The fix was the final commit there...

Cheers,
Rusty.
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