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

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

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 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)))
-- 
1.8.3.1


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