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Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
References: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com>
 <20160115143434.GA25332@blaptop.local> <56991514.9000609@suse.cz>
 <20160116040913.GA566@swordfish>
Cc: Minchan Kim <minchan@kernel.org>, Junil Lee <junil0814.lee@lge.com>,
        ngupta@vflare.org, sergey.senozhatsky.work@gmail.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <5699F4C9.1070902@suse.cz>
Date: Sat, 16 Jan 2016 08:44:09 +0100
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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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