[PATCH] powerpc/pseries: Fix cpu_hotplug_lock acquisition in resize_hpt

From: "Gautham R. Shenoy" <redacted>

During a memory hotplug operations involving resizing of the HPT, we
invoke a stop_machine() to perform the resizing. In this code path, we
end up recursively taking the cpu_hotplug_lock, first in
memory_hotplug_begin() and then subsequently in stop_machine(). This
causes the system to hang. With lockdep enabled we get the following
error message before the hang.

  swapper/0/1 is trying to acquire lock:
  (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: stop_machine+0x2c/0x60

  but task is already holding lock:
  (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: mem_hotplug_begin+0x20/0x50

  other info that might help us debug this:
   Possible unsafe locking scenario:

         CPU0
         ----
    lock(cpu_hotplug_lock.rw_sem);
    lock(cpu_hotplug_lock.rw_sem);

   *** DEADLOCK ***

Fix this issue by
  1) Requiring all the calls to pseries_lpar_resize_hpt() be made
     with cpu_hotplug_lock held.

  2) In pseries_lpar_resize_hpt() invoke stop_machine_cpuslocked()
     as a consequence of 1)

  3) To satisfy 1), in hpt_order_set(), call mmu_hash_ops.resize_hpt()
     with cpu_hotplug_lock held.

Reported-by: Aneesh Kumar K.V <redacted>
Signed-off-by: Gautham R. Shenoy <redacted>
---
 arch/powerpc/mm/hash_utils_64.c       | 9 ++++++++-
 arch/powerpc/platforms/pseries/lpar.c | 8 ++++++--
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index 0a4f939..b05c79c 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c

@@ -37,6 +37,7 @@
 #include <linux/context_tracking.h>
 #include <linux/libfdt.h>
 #include <linux/pkeys.h>
+#include <linux/cpu.h>
 
 #include <asm/debugfs.h>
 #include <asm/processor.h>

@@ -1890,10 +1891,16 @@ static int hpt_order_get(void *data, u64 *val)
 
 static int hpt_order_set(void *data, u64 val)
 {
+	int ret;
+
 	if (!mmu_hash_ops.resize_hpt)
 		return -ENODEV;
 
-	return mmu_hash_ops.resize_hpt(val);
+	cpus_read_lock();
+	ret = mmu_hash_ops.resize_hpt(val);
+	cpus_read_unlock();
+
+	return ret;
 }
 
 DEFINE_DEBUGFS_ATTRIBUTE(fops_hpt_order, hpt_order_get, hpt_order_set, "%llu\n");

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index f2a9f0a..65df95b 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c

@@ -859,7 +859,10 @@ static int pseries_lpar_resize_hpt_commit(void *data)
 	return 0;
 }
 
-/* Must be called in user context */
+/*
+ * Must be called in user context. The caller should hold the
+ * cpus_lock.
+ */
 static int pseries_lpar_resize_hpt(unsigned long shift)
 {
 	struct hpt_resize_state state = {

@@ -911,7 +914,8 @@ static int pseries_lpar_resize_hpt(unsigned long shift)
 
 	t1 = ktime_get();
 
-	rc = stop_machine(pseries_lpar_resize_hpt_commit, &state, NULL);
+	rc = stop_machine_cpuslocked(pseries_lpar_resize_hpt_commit,
+				     &state, NULL);
 
 	t2 = ktime_get();
 

-- 
1.9.4