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

["Zhang, Yang Z" <yang.z.zhang@xxxxxxxxx>]

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@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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