linux-kernel - Re: [PATCH] kdump: Fix crash_kexec

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Date:	Mon, 31 Oct 2011 03:39:48 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	holzheu@...ux.vnet.ibm.com
Cc:	heiko.carstens@...ibm.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	schwidefsky@...ibm.com, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] kdump: Fix crash_kexec - smp_send_stop race in panic

On Mon, 31 Oct 2011 10:57:16 +0100 Michael Holzheu <holzheu@...ux.vnet.ibm.com> wrote:

> > Should this be done earlier in the function?  As it stands we'll have
> > multiple CPUs scribbling on buf[] at the same time and all trying to
> > print the same thing at the same time, dumping their stacks, etc. 
> > Perhaps it would be better to single-thread all that stuff
> 
> My fist patch took the spinlock at the beginning of panic(). But then
> Eric asked, if it wouldn't be better to get both panic printk's and I
> agreed.

Hm, why?  It will make a big mess.

> > Also...  this patch affects all CPU architectures, all configs, etc. 
> > So we're expecting that every architecture's smp_send_stop() is able to
> > stop a CPU which is spinning in spin_lock(), possibly with local
> > interrupts disabled.  Will this work?
> 
> At least on s390 it will work. If there are architectures that can't
> stop disabled CPUs then this problem is already there without this
> patch.
> 
> Example:
> 
> 1. 1st CPU gets lock X and panics
> 2. 2nd CPU is disabled and gets lock X

(irq-disabled)

> 3. 1st CPU calls smp_send_stop()
>    -> 2nd CPU loops disabled and can't be stopped

Well OK.  Maybe some architectures do have this problem - who would
notice?  If that is the case, we just made the failure cases much more
common.  Could you check, please?

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