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

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Date:	Sat, 25 Jan 2014 22:44:21 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.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.

"Zhang, Yang Z" <yang.z.zhang@...el.com> wrote:
>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.
>

No. It needs rework.
>> 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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