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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

"Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>  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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