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Subject[RESEND PATCH] powerpc/pseries: Fix cpu_hotplug_lock acquisition in resize_hpt
Date
From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>

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 <aneesh.kumar@linux.ibm.com>
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
---

Rebased this one against powerpc/next instead of linux/master.

arch/powerpc/mm/book3s64/hash_utils.c | 9 ++++++++-
arch/powerpc/platforms/pseries/lpar.c | 8 ++++++--
2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index 919a861..d07fcafd 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c
@@ -38,6 +38,7 @@
#include <linux/libfdt.h>
#include <linux/pkeys.h>
#include <linux/hugetlb.h>
+#include <linux/cpu.h>

#include <asm/debugfs.h>
#include <asm/processor.h>
@@ -1928,10 +1929,16 @@ 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 1034ef1..2fc9756 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 = {
@@ -913,7 +916,8 @@ 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
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