[PATCH for-4.19 2/9] xen/cpu: do not get the CPU map in stop_machine_run()

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The current callers of stop_machine_run() outside of init code already have the
CPU maps locked, and hence there's no reason for stop_machine_run() to attempt
to lock again.

Replace the get_cpu_maps() call with a suitable unreachable assert.

Further changes will modify the conditions under which get_cpu_maps() returns
success and without the adjustment proposed here the usage of
stop_machine_run() in cpu_down() would then return an error.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/common/cpu.c          |  5 +++++
 xen/common/stop_machine.c | 15 ++++++++-------
 xen/include/xen/cpu.h     |  2 ++
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/xen/common/cpu.c b/xen/common/cpu.c
index 8709db4d2957..6173220e771b 100644
--- a/xen/common/cpu.c
+++ b/xen/common/cpu.c
@@ -68,6 +68,11 @@ void cpu_hotplug_done(void)
     write_unlock(&cpu_add_remove_lock);
 }
 
+bool cpu_map_locked(void)
+{
+    return rw_is_locked(&cpu_add_remove_lock);
+}
+
 static NOTIFIER_HEAD(cpu_chain);
 
 void __init register_cpu_notifier(struct notifier_block *nb)
diff --git a/xen/common/stop_machine.c b/xen/common/stop_machine.c
index 398cfd507c10..7face75648e8 100644
--- a/xen/common/stop_machine.c
+++ b/xen/common/stop_machine.c
@@ -82,9 +82,15 @@ int stop_machine_run(int (*fn)(void *data), void *data, 
unsigned int cpu)
     BUG_ON(!local_irq_is_enabled());
     BUG_ON(!is_idle_vcpu(current));
 
-    /* cpu_online_map must not change. */
-    if ( !get_cpu_maps() )
+    /*
+     * cpu_online_map must not change.  The only two callers of
+     * stop_machine_run() outside of init code already have the CPU map locked.
+     */
+    if ( system_state >= SYS_STATE_active && !cpu_map_locked() )
+    {
+        ASSERT_UNREACHABLE();
         return -EBUSY;
+    }
 
     nr_cpus = num_online_cpus();
     if ( cpu_online(this) )
@@ -92,10 +98,7 @@ int stop_machine_run(int (*fn)(void *data), void *data, 
unsigned int cpu)
 
     /* Must not spin here as the holder will expect us to be descheduled. */
     if ( !spin_trylock(&stopmachine_lock) )
-    {
-        put_cpu_maps();
         return -EBUSY;
-    }
 
     stopmachine_data.fn = fn;
     stopmachine_data.fn_data = data;
@@ -136,8 +139,6 @@ int stop_machine_run(int (*fn)(void *data), void *data, 
unsigned int cpu)
 
     spin_unlock(&stopmachine_lock);
 
-    put_cpu_maps();
-
     return ret;
 }
 
diff --git a/xen/include/xen/cpu.h b/xen/include/xen/cpu.h
index e1d4eb59675c..d8c8264c58b0 100644
--- a/xen/include/xen/cpu.h
+++ b/xen/include/xen/cpu.h
@@ -13,6 +13,8 @@ void put_cpu_maps(void);
 void cpu_hotplug_begin(void);
 void cpu_hotplug_done(void);
 
+bool cpu_map_locked(void);
+
 /* Receive notification of CPU hotplug events. */
 void register_cpu_notifier(struct notifier_block *nb);
 
-- 
2.44.0