[PATCH] xen: Fix XenStore initialisation for XS_LOCAL

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

In commit 3499ba8198ca ("xen: Fix event channel callback via INTX/GSI")
I reworked the triggering of xenbus_probe().

I tried to simplify things by taking out the workqueue based startup
triggered from wake_waiting(); the somewhat poorly named xenbus IRQ
handler.

I missed the fact that in the XS_LOCAL case (Dom0 starting its own
xenstored or xenstore-stubdom, which happens after the kernel is booted
completely), that IRQ-based trigger is still actually needed.

So... put it back, except more cleanly. By just spawning a xenbus_probe
thread which waits on xb_waitq and runs the probe the first time it
gets woken, just as the workqueue-based hack did.

This is actually a nicer approach for *all* the back ends with different
interrupt methods, and we can switch them all over to that without the
complex conditions for when to trigger it. But not in -rc6. This is
the minimal fix for the regression, although it's a step in the right
direction instead of doing a partial revert and actually putting the
workqueue back. It's also simpler than the workqueue.

Fixes: 3499ba8198ca ("xen: Fix event channel callback via INTX/GSI")
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 drivers/xen/xenbus/xenbus_probe.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe.c 
b/drivers/xen/xenbus/xenbus_probe.c
index c8f0282bb649..18ffd0551b54 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -714,6 +714,23 @@ static bool xs_hvm_defer_init_for_callback(void)
 #endif
 }
 
+static int xenbus_probe_thread(void *unused)
+{
+       DEFINE_WAIT(w);
+
+       /*
+        * We actually just want to wait for *any* trigger of xb_waitq,
+        * and run xenbus_probe() the moment it occurs.
+        */
+       prepare_to_wait(&xb_waitq, &w, TASK_INTERRUPTIBLE);
+       schedule();
+       finish_wait(&xb_waitq, &w);
+
+       DPRINTK("probing");
+       xenbus_probe();
+       return 0;
+}
+
 static int __init xenbus_probe_initcall(void)
 {
        /*
@@ -725,6 +742,20 @@ static int __init xenbus_probe_initcall(void)
             !xs_hvm_defer_init_for_callback()))
                xenbus_probe();
 
+       /*
+        * For XS_LOCAL, spawn a thread which will wait for xenstored
+        * or a xenstore-stubdom to be started, then probe. It will be
+        * triggered when communication starts happening, by waiting
+        * on xb_waitq.
+        */
+       if (xen_store_domain_type == XS_LOCAL) {
+               struct task_struct *probe_task;
+
+               probe_task = kthread_run(xenbus_probe_thread, NULL,
+                                        "xenbus_probe");
+               if (IS_ERR(probe_task))
+                       return PTR_ERR(probe_task);
+       }
        return 0;
 }
 device_initcall(xenbus_probe_initcall);
-- 
2.29.2

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