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

[Konrad Rzeszutek Wilk]

On Fri, Apr 04, 2014 at 04:35:27PM -0400, Boris Ostrovsky wrote:
> On 04/04/2014 02:53 PM, 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.
> 
> This looks like a runaway sentence.

I am not sure what I thought when I wrote that. It should just
go away. Thanks for noticing it!

> 
> Other than that
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> 
> >
> >Fixes-Bug: http://bugs.xenproject.org/xen/bug/8
> >Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >[v2: Fixed it up per David's suggestions]
> >Reviewed-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> >---
> >  drivers/xen/xenbus/xenbus_xs.c | 44 +++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 41 insertions(+), 3 deletions(-)
> >
> >diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
> >index b6d5fff..ba804f3 100644
> >--- a/drivers/xen/xenbus/xenbus_xs.c
> >+++ b/drivers/xen/xenbus/xenbus_xs.c
> >@@ -50,6 +50,7 @@
> >  #include <xen/xenbus.h>
> >  #include <xen/xen.h>
> >  #include "xenbus_comms.h"
> >+#include "xenbus_probe.h"
> >  struct xs_stored_msg {
> >  	struct list_head list;
> >@@ -139,6 +140,29 @@ static int get_error(const char *errorstring)
> >  	return xsd_errors[i].errnum;
> >  }
> >+static bool xenbus_ok(void)
> >+{
> >+	switch (xen_store_domain_type) {
> >+	case XS_LOCAL:
> >+		switch (system_state) {
> >+		case SYSTEM_POWER_OFF:
> >+		case SYSTEM_RESTART:
> >+		case SYSTEM_HALT:
> >+			return false;
> >+		default:
> >+			break;
> >+		}
> >+		return true;
> >+	case XS_PV:
> >+	case XS_HVM:
> >+		/* FIXME: Could check that the remote domain is alive,
> >+		 * but it is normally initial domain. */
> >+		return true;
> >+	default:
> >+		break;
> >+	}
> >+	return false;
> >+}
> >  static void *read_reply(enum xsd_sockmsg_type *type, unsigned int *len)
> >  {
> >  	struct xs_stored_msg *msg;
> >@@ -148,9 +172,20 @@ 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));
> >+		if (xenbus_ok())
> >+			/* XXX FIXME: Avoid synchronous wait for response here. */
> >+			wait_event_timeout(xs_state.reply_waitq,
> >+					   !list_empty(&xs_state.reply_list),
> >+					   msecs_to_jiffies(500));
> >+		else {
> >+			/*
> >+			 * 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.
> >+			 */
> >+			return ERR_PTR(-EIO);
> >+		}
> >  		spin_lock(&xs_state.reply_lock);
> >  	}
> >@@ -215,6 +250,9 @@ 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)))
> 
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