linux-kernel - RE: [RFC][PATCH v2 -next 2/2] Adding lock operations to kmsg_dump()/pstore

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Message-ID: <987664A83D2D224EAE907B061CE93D5301F2030C4F@orsmsx505.amr.corp.intel.com>
Date:	Fri, 28 Oct 2011 12:02:15 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Don Zickus <dzickus@...hat.com>,
	Seiji Aguchi <seiji.aguchi@....com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"Chen, Gong" <gong.chen@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Matthew Garrett <mjg@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	"Brown, Len" <len.brown@...el.com>,
	"Huang, Ying" <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 v2 -next 2/2] Adding lock operations to
 kmsg_dump()/pstore_dump()

> It ain't pretty but it moves things towards a more reliable message dump.
> The odds of us needing to bust the spinlocks are really small.  Most of
> the time no one reads the pstore filesystem.

Does it really change the odds much?  As you say, the common case is that
pstore front-end doesn't have the lock held - so that case is unchanged.
We can get the lock anyway, we don't need to bust it.

Looking at the uncommon case where the lock is held - that means that
pstore was in the middle of some back-end operation. Busting the lock
means that the back-end will be surprised by being called again when the
first operation had not yet completed. In the case of a state machine
driven back end like ERST, I don't think this has a high probability of
working out well.

So you might be moving the needle from 99.999% chance of saving to pstore
with 0.001% chance of hanging on the spin lock. to 99.9991% chance of
saving, and 0.0009% chance of something highly weird happening in the
back-end driver because you busted the lock and called it anyway.

> I would love to figure out a prettier solution for this locking mess, but
> I can't think of anything.  We have customers who want to utilize this
> technology, so I am trying to make sure it is stable and robust for now.
> A little selfish I suppose.  But we are open to ideas?

If a prettier solution is needed - it will have to involve the back-end.
Perhaps a whole separate write/panic path (with separate buffer). Then
a sufficiently smart back end could do the right thing. I have little
confidence that ERST could be made smart in this way, because almost all
of the heavy lifting is done by the BIOS - so Linux has no way to influence
the flow of execution.

-Tony
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