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

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Date:	Mon, 18 Jan 2016 16:04:27 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	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

Hello,

oh, you replied in this thread.

On (01/18/16 15:32), Minchan Kim wrote:
> >                 free_obj = obj_malloc(d_page, class, handle);
> >                 zs_object_copy(free_obj, used_obj, class);
> >                 index++;
> > +               /* This also effectively unpins the handle */
> >                 record_obj(handle, free_obj);
> > -               unpin_tag(handle);
> >                 obj_free(pool, class, used_obj);
> >         }
> > 
> > But I'd still recommend WRITE_ONCE in record_obj(). And I'm not even sure it's
> 
> Thanks for the reivew. Yeah, we need WRITE_ONCE in record_obj but
> your version will not work. IMHO, WRITE_ONCE can prevent store-tearing
> but it couldn't prevent reordering. IOW, we need some barrier as unlock
> and clear_bit_unlock includes it.
> So, we shouldn't omit unpin_tag there.

but there is only one store operation after this patch.

static void record_obj(unsigned long handle, unsigned long obj)
{
	*(unsigned long *)handle = obj;
}

does the re-ordering problem exist? zs_free() will see the
old pinned handle and spin, until record_obj() from migrate.

	-ss

> > safe on all architectures to do a simple overwrite of a word against somebody
> > else trying to lock a bit there?
> 
> Hmm, I think it shouldn't be a problem. It's word-alinged, word-sized
> store so it should be atomic.
> 
> As other example, we have been used lock_page for a bit of page->flags
> and used other bits in there with __set_bit(ie, __SetPageXXX).
> I guess it's same situation with us just except we are spinning there.
> But it is worth to dobule check so need to help lock guys.