linux-kernel - Re: [PATCH 1/2] xen/xenbus: Avoid synchronous wait on XenBus stalling shutdown/restart.

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Date:	Mon, 7 Apr 2014 18:17:59 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<boris.ostrovsky@...cle.com>, <xen-devel@...ts.xenproject.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] xen/xenbus: Avoid synchronous wait on XenBus stalling
 shutdown/restart.

On 04/04/14 19:53, Konrad Rzeszutek Wilk wrote:
> The 'read_reply' works with 'process_msg' to read of a reply in XenBus.
> 'process_msg' is running from within the 'xenbus' thread. Whenever
> a message shows up in XenBus it is put on a xs_state.reply_list list
> and 'read_reply' picks it up.
> 
> The problem is if the backend domain or the xenstored process is killed.
> In which case 'xenbus' is still awaiting - and 'read_reply' if called -
> stuck forever waiting for the reply_list to have some contents.
> 
> This is normally not a problem - as the backend domain can come back
> or the xenstored process can be restarted. However if the domain
> is in process of being powered off/restarted/halted - there is no
> point of waiting on it coming back - as we are effectively being
> terminated and should not impede the progress.
> 
> This patch solves this problem by checking whether the guest is
> the right domain. If it is an initial domain and hurtling towards
> death - there is no point of continuing the wait. All other type
> of guests continue with their behavior.
> mechanism a bit more asynchronous.

Applied to devel/for-linus-3.15.

I rewrote this last paragraph to:

"This patch solves this problem by checking whether the guest is the
right domain. If it is an initial domain and hurtling towards death -
there is no point of continuing the wait. All other type of guests
continue with their behavior (as Xenstore is expected to still be
running in another domain)."

David
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