linux-kernel - Re: [RFC][PATCH] pstore: Skip spinlock when just one cpu is online

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Date:	Mon, 10 Dec 2012 11:48:13 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Seiji Aguchi <seiji.aguchi@....com>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cbouatmailru@...il.com" <cbouatmailru@...il.com>,
	"ccross@...roid.com" <ccross@...roid.com>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Satoru Moriya <satoru.moriya@....com>
Subject: Re: [RFC][PATCH] pstore: Skip spinlock when just one cpu is online

On Fri, Dec 07, 2012 at 11:43:03PM +0000, Seiji Aguchi wrote:
> > Can all these things really happen (did you run into this problem on a real system?). Or is this just a theoretical problem.  Ugly (but
> > practical) hacks might be OK to solve real problems. 
> 
> It is a theoretical problem right now.
> But it is a timing issue and there is a possibility to happen actually.
> 
> > But do we really want them to fix problems that actually never happen?
> 
> If we find a problem (even if it is theoretical), we can't say "It actually never happen.".
> 
> I have some reasons to submit this patch before reproducing actually.
> 
> 1)
> It is too late if we fix a problem after it actually happened in case where we apply Linux, including pstore, 
> to mission critical systems, because the failure of those systems has a great impact on a whole society.
> Customers in this area ask us to fix a problem as soon as possible.
> On the other hand, this kind of timing issue is hard to reproduce.
> So, our support service engineers often work all night to reproduce it.
> It is a nightmare for us.
> 
> If we can fix it with a small patch in adance, it is really helpful for us.

As I said in my email I just sent, it may not help you without testing it.
As there are probably other problems in that un-tested theoretical
scenario.

> 
> 2)
> In the long term, I plan to add a kmsg_dump to a kexec path because kdump may fail in the real world.
> In that case, we need another troubleshooting material like pstore to detect a root cause of failure.

But you are assuming that kmsg_dump is perfect and it isn't, in which case
by putting kmsg_dump in the kdump path, you actually may be blocking kdump
from working.

That is the biggest hold up for those guys from including it I believe.

Cheers,
Don
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