linux-kernel - Re: [Xen-devel] [PATCH 1/3] xen/pv-on-hvm kexec: prevent crash in xenwatch

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Date:	Wed, 17 Aug 2011 14:51:27 +0200
From:	Olaf Hering <olaf@...fle.de>
To:	Ian Campbell <Ian.Campbell@...rix.com>,
	Keir Fraser <keir.xen@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Konrad <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [Xen-devel] [PATCH 1/3] xen/pv-on-hvm kexec: prevent crash in
 xenwatch_thread() when stale watch events arrive

On Tue, Aug 16, Ian Campbell wrote:

> On Tue, 2011-08-16 at 14:16 +0100, Olaf Hering wrote:
> > During repeated kexec boots xenwatch_thread() can crash because
> > xenbus_watch->callback is cleared by xenbus_watch_path() if a node/token
> > combo for a new watch happens to match an already registered watch from
> > an old kernel.  In this case xs_watch returns -EEXISTS, then
> > register_xenbus_watch() does not remove the to-be-registered watch from
> > the list of active watches but returns the -EEXISTS to the caller
> > anyway.
> 
> Isn't this behaviour the root cause of the issue (which should be fixed)
> rather than papering over it during watch processing. IOW should't
> register_xenbus_watch cleanup after itself if xs_watch fails.

Keir, the EEXISTS case in register_xenbus_watch() was added by you 6
years ago. Do you happen to know what it tried to solve, and do these
conditions still apply today?  Perhaps the EEXISTS can be removed now.

http://xenbits.xen.org/hg/xen-unstable.hg/diff/8016551fde98/linux-2.6-xen-sparse/drivers/xen/xenbus/xenbus_xs.c

Olaf
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