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

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Date:	Sat, 16 Jan 2016 13:09:13 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
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,
	sergey.senozhatsky@...il.com
Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition

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);
	}

	-ss