[PATCH] mm/hotplug: Remove stop_machine() from try_offline_node()

[Toshi Kani]

lock_device_hotplug() serializes hotplug & online/offline operations.
The lock is held in common sysfs online/offline interfaces and ACPI
hotplug code paths.

try_offline_node() off-lines a node if all memory sections and cpus
are removed on the node.  It is called from acpi_processor_remove()
and acpi_memory_remove_memory()->remove_memory() paths, both of which
are in the ACPI hotplug code.

try_offline_node() calls stop_machine() to stop all cpus while checking
all cpu status with the assumption that the caller is not protected from
CPU hotplug or CPU online/offline operations.  However, the caller is
always serialized with lock_device_hotplug().  Also, the code needs to
be properly serialized with a lock, not by stopping all cpus at a random
place with stop_machine().

This patch removes the use of stop_machine() in try_offline_node() and
adds comments to try_offline_node() and remove_memory() that
lock_device_hotplug() is required.

Signed-off-by: Toshi Kani <toshi.kani@xxxxxx>
---
 mm/memory_hotplug.c |   31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index ca1dd3a..0b4b0f7 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1674,9 +1674,8 @@ static int is_memblock_offlined_cb(struct memory_block *mem, void *arg)
 	return ret;
 }
 
-static int check_cpu_on_node(void *data)
+static int check_cpu_on_node(pg_data_t *pgdat)
 {
-	struct pglist_data *pgdat = data;
 	int cpu;
 
 	for_each_present_cpu(cpu) {
@@ -1691,10 +1690,9 @@ static int check_cpu_on_node(void *data)
 	return 0;
 }
 
-static void unmap_cpu_on_node(void *data)
+static void unmap_cpu_on_node(pg_data_t *pgdat)
 {
 #ifdef CONFIG_ACPI_NUMA
-	struct pglist_data *pgdat = data;
 	int cpu;
 
 	for_each_possible_cpu(cpu)
@@ -1703,10 +1701,11 @@ static void unmap_cpu_on_node(void *data)
 #endif
 }
 
-static int check_and_unmap_cpu_on_node(void *data)
+static int check_and_unmap_cpu_on_node(pg_data_t *pgdat)
 {
-	int ret = check_cpu_on_node(data);
+	int ret;
 
+	ret = check_cpu_on_node(pgdat);
 	if (ret)
 		return ret;
 
@@ -1715,11 +1714,18 @@ static int check_and_unmap_cpu_on_node(void *data)
 	 * the cpu_to_node() now.
 	 */
 
-	unmap_cpu_on_node(data);
+	unmap_cpu_on_node(pgdat);
 	return 0;
 }
 
-/* offline the node if all memory sections of this node are removed */
+/**
+ * try_offline_node
+ *
+ * Offline a node if all memory sections and cpus of the node are removed.
+ *
+ * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
+ * and online/offline operations before this call.
+ */
 void try_offline_node(int nid)
 {
 	pg_data_t *pgdat = NODE_DATA(nid);
@@ -1745,7 +1751,7 @@ void try_offline_node(int nid)
 		return;
 	}
 
-	if (stop_machine(check_and_unmap_cpu_on_node, pgdat, NULL))
+	if (check_and_unmap_cpu_on_node(pgdat))
 		return;
 
 	/*
@@ -1782,6 +1788,13 @@ void try_offline_node(int nid)
 }
 EXPORT_SYMBOL(try_offline_node);
 
+/**
+ * remove_memory
+ *
+ * NOTE: The caller must call lock_device_hotplug() to serialize hotplug
+ * and online/offline operations before this call, as required by
+ * try_offline_node().
+ */
 void __ref remove_memory(int nid, u64 start, u64 size)
 {
 	int ret;
--
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