linux-kernel - Re: [RFC][PATCH v3 2/3] Skip spin_locks in panic case and add WARN

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Date:	Mon, 19 Dec 2011 16:00:46 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Seiji Aguchi <seiji.aguchi@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Matthew Garrett <mjg@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"Chen, Gong" <gong.chen@...el.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"Brown, Len" <len.brown@...el.com>,
	"'ying.huang@...el.com'" <'ying.huang@...el.com'>,
	"'ak@...ux.intel.com'" <'ak@...ux.intel.com'>,
	"'hughd@...omium.org'" <'hughd@...omium.org'>,
	"'mingo@...e.hu'" <'mingo@...e.hu'>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"namhyung@...il.com" <namhyung@...il.com>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Satoru Moriya <satoru.moriya@....com>
Subject: Re: [RFC][PATCH v3 2/3] Skip spin_locks in panic case and add
 WARN_ON()

On Mon, Dec 12, 2011 at 10:48:52AM -0800, Luck, Tony wrote:
> > if (reason == KMSG_DUMP_PANIC) {
> > 	if(is_spin_locked(&psinfo->buf_lock))
> > 		pr_err("lock is taken.\n");
> > else {
> > 	spin_lock_irqsave(&psinfo->buf_lock, flags);
> > }
> >
> > However, this won't work for this reason.
> >  - printk() must not be called in serialized path because deadlock of logbuf_lock may cause.
> 
> There is also the issue that kmsg has already computed the addresses
> of useful pieces in __log_buf at this point - so any printk() we
> add here is not going to be saved into pstore.  So a user relying
> on pstore to see what happened, won't see any messages we add here.

Ah.  Good point.

Now we have a scenario (though rare), where kmsg_dump can fail to save a
dump because some one already has the lock.  Hitachi's whole goal behind
kmsg_dump is to record every possible reason for rebooting the machine for
inspection on reboot, I think if I understood Seiji privately.

Seiji, has does this failure impact kmsg_dump's goal?

Cheers,
Don
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