linux-kernel - Re: [PATCH 2/2] mm/swap: Remove lock_initialized flag from swap_slots

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Message-ID: <e6445164-ab4b-86cc-731f-5f6509a7449d@linux.intel.com>
Date:   Mon, 24 Jul 2017 09:54:50 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Wenwei Tao <wenwei.tww@...baba-inc.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>
Subject: Re: [PATCH 2/2] mm/swap: Remove lock_initialized flag from
 swap_slots_cache

On 07/23/2017 07:15 PM, Huang, Ying wrote:
> Hi, Tim,
> 
> Tim Chen <tim.c.chen@...ux.intel.com> 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