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Subject[RFC PATCH 3/4] Hyper-V/Balloon: Call add_memory() with dm_device.ha_lock.
Date
From: Tianyu Lan <Tianyu.Lan@microsoft.com>

The ha_lock is to protect hot-add region list ha_region_list.
When Hyper-V delivers hot-add memory message, handle_pg_range()
goes through the list to find the hot-add region state
associated with message and do hot-add memory. The lock
is released in the loop before calling hv_mem_hot_add()
and is reacquired in hv_mem_hot_add(). There is a race
that list entry maybe freed during the slot.

To avoid the race and simply the code, make hv_mem_hot_add()
under protection of ha_region_list lock. There is a dead lock
case when run add_memory() under ha_lock. add_memory() calls
hv_online_page() inside and hv_online_page() also acquires
ha_lock again. Add lock_thread in the struct hv_dynmem_device
to record hv_mem_hot_add()'s thread and check lock_thread
in hv_online_page(). hv_mem_hot_add() thread already holds
lock during traverse hot add list and so not acquire lock
in hv_online_page().

Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
---
drivers/hv/hv_balloon.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 34bd73526afd..4d1a3b1e2490 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -545,6 +545,7 @@ struct hv_dynmem_device {
* regions from ha_region_list.
*/
spinlock_t ha_lock;
+ struct task_struct *lock_thread;

/*
* A list of hot-add regions.
@@ -707,12 +708,10 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
unsigned long start_pfn;
unsigned long processed_pfn;
unsigned long total_pfn = pfn_count;
- unsigned long flags;

for (i = 0; i < (size/HA_CHUNK); i++) {
start_pfn = start + (i * HA_CHUNK);

- spin_lock_irqsave(&dm_device.ha_lock, flags);
has->ha_end_pfn += HA_CHUNK;

if (total_pfn > HA_CHUNK) {
@@ -724,7 +723,6 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
}

has->covered_end_pfn += processed_pfn;
- spin_unlock_irqrestore(&dm_device.ha_lock, flags);

init_completion(&dm_device.ol_waitevent);
dm_device.ha_waiting = !memhp_auto_online;
@@ -745,10 +743,8 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
*/
do_hot_add = false;
}
- spin_lock_irqsave(&dm_device.ha_lock, flags);
has->ha_end_pfn -= HA_CHUNK;
has->covered_end_pfn -= processed_pfn;
- spin_unlock_irqrestore(&dm_device.ha_lock, flags);
break;
}

@@ -771,8 +767,13 @@ static void hv_online_page(struct page *pg, unsigned int order)
struct hv_hotadd_state *has;
unsigned long flags;
unsigned long pfn = page_to_pfn(pg);
+ int unlocked;
+
+ if (dm_device.lock_thread != current) {
+ spin_lock_irqsave(&dm_device.ha_lock, flags);
+ unlocked = 1;
+ }

- spin_lock_irqsave(&dm_device.ha_lock, flags);
list_for_each_entry(has, &dm_device.ha_region_list, list) {
/* The page belongs to a different HAS. */
if ((pfn < has->start_pfn) ||
@@ -782,7 +783,9 @@ static void hv_online_page(struct page *pg, unsigned int order)
hv_bring_pgs_online(has, pfn, 1UL << order);
break;
}
- spin_unlock_irqrestore(&dm_device.ha_lock, flags);
+
+ if (unlocked)
+ spin_unlock_irqrestore(&dm_device.ha_lock, flags);
}

static int pfn_covered(unsigned long start_pfn, unsigned long pfn_cnt)
@@ -860,6 +863,7 @@ static unsigned long handle_pg_range(unsigned long pg_start,
pg_start);

spin_lock_irqsave(&dm_device.ha_lock, flags);
+ dm_device.lock_thread = current;
list_for_each_entry(has, &dm_device.ha_region_list, list) {
/*
* If the pfn range we are dealing with is not in the current
@@ -912,9 +916,7 @@ static unsigned long handle_pg_range(unsigned long pg_start,
} else {
pfn_cnt = size;
}
- spin_unlock_irqrestore(&dm_device.ha_lock, flags);
hv_mem_hot_add(has->ha_end_pfn, size, pfn_cnt, has);
- spin_lock_irqsave(&dm_device.ha_lock, flags);
}
/*
* If we managed to online any pages that were given to us,
@@ -923,6 +925,7 @@ static unsigned long handle_pg_range(unsigned long pg_start,
res = has->covered_end_pfn - old_covered_state;
break;
}
+ dm_device.lock_thread = NULL;
spin_unlock_irqrestore(&dm_device.ha_lock, flags);

return res;
--
2.14.5
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