Re: [PATCH 2/2] mm/swap: Remove lock_initialized flag from swap_slots_cache
From: Tim Chen
Date: Mon Jul 24 2017 - 12:55:29 EST
On 07/23/2017 07:15 PM, Huang, Ying wrote:
> Hi, Tim,
>
> Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> writes:
>
>> We will only reach the lock initialization code
>> in alloc_swap_slot_cache when the cpu's swap_slots_cache's slots
>> have not been allocated and swap_slots_cache has not been initialized
>> previously. So the lock_initialized check is redundant and unnecessary.
>> Remove lock_initialized flag from swap_slots_cache to save memory.
>
> Is there a race condition with CPU offline/online when preempt is enabled?
>
> CPU A CPU B
> ----- -----
> get_swap_page()
> get cache[B], cache[B]->slots != NULL
> preempted and moved to CPU A
> be offlined
> be onlined
> alloc_swap_slot_cache()
> mutex_lock(cache[B]->alloc_lock)
> mutex_init(cache[B]->alloc_lock) !!!
>
> The cache[B]->alloc_lock will be reinitialized when it is still held.
Looks like for this case the lock_initialized flag is still needed
to prevent such races and prevent re-initialization of taken locks.
Okay, let's scrap patch 2.
Thanks.
Tim