linux-kernel - [Question] Is there race between swapoff and swapout

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Message-ID: <8860e8b1-7bad-0b4a-60d1-4893973b9cb2@huawei.com>
Date:   Mon, 14 Oct 2019 21:22:35 +0800
From:   Chen Wandun <chenwandun@...wei.com>
To:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <ying.huang@...el.com>
Subject: [Question] Is there race between swapoff and swapout

I analysied the code about swapoff and swapout, and I suspected there may be a race.
The kernel version is 4.14 stable.

CPU0						CPU1
swapoff						swap out
						add_to_swap
							get_swap_page
	......							get_swap_pages						
									spin_lock(&swap_avail_lock)
									get swap_info_struct
									spin_unlock(&swap_avail_lock)		
	spin_lock(&swap_avail_lock)									
	__def_from_avail_list(swap_info_struct)								
     	spin_unlock(&swap_avail_lock)					......	
	try_to_unuse  // unuse all slot
									/* get a free slot from swap_info_struct,
									 * and write data to slot later
									 */	
									scan_swap_map_slots
	free swap_info_struct
	.......

	
If CPU1 get the swap_info_struct first, then CPU0 delete it from list and
unuse all slot in swap_info_struct, before CPU0 free swap_info_struct CPU1
call scan_swap_map_slots to alloc a free slot.

I am not sure the analysis above is correct,
Please let me know if there is any mistake

Thanks
ChenWandun