xen: fix hang on suspend.

In 65f63384 "xen: improve error handling in do_suspend" I said:
    - xs_suspend()/xs_resume() and dpm_suspend_noirq()/dpm_resume_noirq() were not
      nested in the obvious way.
and changed the ordering of the calls as so:
    BEFORE		AFTER
    xs_suspend		dpm_suspend_noirq
    dpm_suspend_noirq	xs_suspend
    *SUSPEND*		*SUSPEND*
    dpm_resume_noirq	dpm_resume_noirq
    xs_resume		xs_resume
Clearly this is not an improvement and I was talking rubbish.

In particular the new ordering is susceptible to a hang if a xenstore write is
in progress at the point at which the suspend kicks in. When the suspend
process calls xs_suspend it tries to take the request_mutex but if a write is
in progress it could be looping in xenbus_xs.c:read_reply() waiting for
something to arrive on &xs_state.reply_list while holding the request_mutex
(taken in the caller of read_reply).

However if we have done dpm_suspend_noirq before xs_suspend then we won't get
any more xenstore interrupts and process_msg() will never be woken up to add
anything to the reply_list.

Fix this by calling xs_suspend before dpm_suspend_noirq. If dpm_suspend_noirq
fails then make sure we go through the xs_suspend_cancel() code path.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stable Kernel <stable@kernel.org>

drivers/xen/manage.c

@@ -102,15 +102,15 @@ static void do_suspend(void)
 		goto out_thaw;
 	}
 
+	printk(KERN_DEBUG "suspending xenstore...\n");
+	xs_suspend();
+
 	err = dpm_suspend_noirq(PMSG_SUSPEND);
 	if (err) {
 		printk(KERN_ERR "dpm_suspend_noirq failed: %d\n", err);
 		goto out_resume;
 	}
 
-	printk(KERN_DEBUG "suspending xenstore...\n");
-	xs_suspend();
-
 	err = stop_machine(xen_suspend, &cancelled, cpumask_of(0));
 
 	dpm_resume_noirq(PMSG_RESUME);
@@ -120,13 +120,13 @@ static void do_suspend(void)
 		cancelled = 1;
 	}
 
+out_resume:
 	if (!cancelled) {
 		xen_arch_resume();
 		xs_resume();
 	} else
 		xs_suspend_cancel();
 
-out_resume:
 	dpm_resume_end(PMSG_RESUME);
 
 	/* Make sure timer events get retriggered on all CPUs */