linux-kernel - RE: [Xen-devel] [PATCH 4/4] xen/xenbus: Avoid synchronous wait on XenBus stalling shutdown/restart.

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Date:	Sun, 26 Jan 2014 01:13:51 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"JBeulich@...e.com" <JBeulich@...e.com>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>
Subject: RE: [Xen-devel] [PATCH 4/4] xen/xenbus: Avoid synchronous wait on
	XenBus stalling shutdown/restart.

Konrad Rzeszutek Wilk wrote on 2013-11-09:
> 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.
> 

Hi, Konrad,

Is this patch applied in Linux upstream tree? I didn't find it with latest Linux upstream source.

> This patch solves this problem by checking the 'system_state' value to
> see if we are in heading towards death. We also make the wait
> mechanism a bit more asynchronous.
> 
> Fixes-Bug: http://bugs.xenproject.org/xen/bug/8
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  drivers/xen/xenbus/xenbus_xs.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
> diff --git a/drivers/xen/xenbus/xenbus_xs.c
> b/drivers/xen/xenbus/xenbus_xs.c index b6d5fff..4f22706 100644
> --- a/drivers/xen/xenbus/xenbus_xs.c
> +++ b/drivers/xen/xenbus/xenbus_xs.c
> @@ -148,9 +148,24 @@ static void *read_reply(enum xsd_sockmsg_type
> *type, unsigned int *len)
> 
>  	while (list_empty(&xs_state.reply_list)) {
>  		spin_unlock(&xs_state.reply_lock);
> -		/* XXX FIXME: Avoid synchronous wait for response here. */
> -		wait_event(xs_state.reply_waitq, -			  
> !list_empty(&xs_state.reply_list));
> +		wait_event_timeout(xs_state.reply_waitq, +				  
> !list_empty(&xs_state.reply_list), +				   msecs_to_jiffies(500)); +
> +		/* +		 * If we are in the process of being shut-down there is +		 *
> no point of trying to contact XenBus - it is either +		 * killed
> (xenstored application) or the other domain +		 * has been killed or is
> unreachable. +		 */ +		switch (system_state) { +		case SYSTEM_POWER_OFF:
> +		case SYSTEM_RESTART: +		case SYSTEM_HALT: +			return ERR_PTR(-EIO);
> +		default: +			break; +		}
>  		spin_lock(&xs_state.reply_lock);
>  	}
> @@ -215,6 +230,9 @@ void *xenbus_dev_request_and_reply(struct
> xsd_sockmsg *msg)
> 
>  	mutex_unlock(&xs_state.request_mutex);
> +	if (IS_ERR(ret))
> +		return ret;
> +
>  	if ((msg->type == XS_TRANSACTION_END) ||
>  	    ((req_msg.type == XS_TRANSACTION_START) &&
>  	     (msg->type == XS_ERROR)))


Best regards,
Yang


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