linux-kernel - Re: [patch 1/5] Debugobjects transition check

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Date:	Tue, 6 Apr 2010 10:15:51 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	laijs@...fujitsu.com, dipankar@...ibm.com, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, adobriyan@...il.com,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [patch 1/5] Debugobjects transition check

On Wed, 31 Mar 2010, Mathieu Desnoyers wrote:
> > > where "expect" is the expected state and "next" is the next state to move to if
> > > the expected state is found. A warning is generated if the expected is not
> > > found.
> > 
> > Does it only warn or is there a callback to fixup things as well ?
> 
> For the moment, it only warns. I have not seen the need for a fixup callback
> yet. It might become useful at some point, but I prefer to proceed
> incrementally. This kind of callback could become quite big too, because it
> would have to deal with transitions "from each to each" states of the system,
> with, in the worse case scenario, different fixups for each situation.
> 
> Just for the specific case of "do RCU batch", when detecting that a non-queued
> rcu head is there for execution, there are a few cases to consider:
> 
> - List corruption
>   - Appears in two lists.
>   - Appears in the same list twice.
> - Race (two threads reading the list at the same time).
> - ...
> 
> I am probably forgetting about others. So one way to fixup this would be not to
> execute the callback, but even then, the lists might be corrupted. So it's not
> at all clear to me if we can do much better than reporting the inconsistency
> without increasing intrusiveness. But maybe I just need more imagination. ;)

Yes, that sounds tricky to implement and maybe not worth the
effort. Can you resend the debugobjects patch with a full changelog
please ?

Thanks,

	tglx
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