linux-kernel - Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs

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Date:	Sat, 16 Jan 2016 08:44:09 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Junil Lee <junil0814.lee@....com>, ngupta@...are.org,
	sergey.senozhatsky.work@...il.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition

On 16.1.2016 5:09, Sergey Senozhatsky wrote:
> On (01/15/16 16:49), Vlastimil Babka wrote:
> [..]
>>
>> Could you please also help making the changelog more clear?
>>
>>>
>>>> +		free_obj |= BIT(HANDLE_PIN_BIT);
>>>>  		record_obj(handle, free_obj);
>>
>> I think record_obj() should use WRITE_ONCE() or something like that.
>> Otherwise the compiler is IMHO allowed to reorder this, i.e. first to assign
>> free_obj to handle, and then add the PIN bit there.
> 
> good note.
> 
> ... or do both things in record_obj() (per Minchan)
> 
> 	record_obj(handle, obj)
> 	{
> 	        *(unsigned long)handle = obj & ~(1<<HANDLE_PIN_BIT);

Hmm but that's an unpin, not a pin? A mistake or I'm missing something?
Anyway the compiler can do the same thing here without a WRITE_ONCE().

> 	}
> 
> 	-ss
> 
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