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SubjectRe: [RFC PATCH 3/4] Hyper-V/Balloon: Call add_memory() with dm_device.ha_lock.
Date
lantianyu1986@gmail.com writes:

> 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.

Do I understand correctly that without memory hot remove there's no
race? If yes than we should clarify this in the changelog.

>
> 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) {

With lock_thread checking you're trying to protect against taking the
spinlock twice (when this is called from add_memory()) but why not just
check that spin_is_locked() AND we sit on the same CPU as the VMBus
channel attached to the balloon device?

> + spin_lock_irqsave(&dm_device.ha_lock, flags);
> + unlocked = 1;
> + }

We set unlocked to '1' when we're actually locked, aren't we?

>
> - 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);

Apart from the deadlock you mention in the commit message, add_memory
does lock_device_hotplug()/unlock_device_hotplug() which is a mutex. If
I'm not mistaken you now take the mutext under a spinlock
(&dm_device.ha_lock). Not good.


> }
> /*
> * 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;

--
Vitaly

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