linux-kernel - RE: [PATCH v2] kdump: Fix crash_kexec

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Message-ID: <987664A83D2D224EAE907B061CE93D5301F527F980@orsmsx505.amr.corp.intel.com>
Date:	Wed, 2 Nov 2011 13:57:26 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	"holzheu@...ux.vnet.ibm.com" <holzheu@...ux.vnet.ibm.com>,
	Don Zickus <dzickus@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: RE: [PATCH v2] kdump: Fix crash_kexec - smp_send_stop race in panic

> Instead of introducing the panic lock, as an alternative we could move
> smp_send_stop() to the beginning of panic(). Eric told me that the
> function is currently "insufficiently reliable" for that, but perhaps we
> could make it more reliable.

That's tough to do.  We are in panic because something went horribly
wrong somewhere in the kernel - so we can make few assumptions about
which subsystems are still working. In the worst case (for this example)
our panic was caused by a failure in the code that sends cross-processor
interrupts ... so calling that same code to stop the other cpus is
likely to run into the same problem - perhaps causing a nested panic.

So what looks like a good fix for some panic scenarios actually makes
others worse.

-Tony
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