Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition

[Minchan Kim]

Hello, Vlastimil

On Sat, Jan 16, 2016 at 09:16:41AM +0100, Vlastimil Babka wrote:
> On 16.1.2016 9:06, Sergey Senozhatsky wrote:
> > On (01/16/16 08:44), Vlastimil Babka wrote:
> >> On 16.1.2016 5:09, Sergey Senozhatsky wrote:
> >>> On (01/15/16 16:49), Vlastimil Babka wrote:
> >>
> >> Hmm but that's an unpin, not a pin? A mistake or I'm missing something?
> > 
> > I'm sure it's just a compose-in-mail-app typo.
> 
> BTW, couldn't the correct fix also just look like this?
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 9f15bdd9163c..43f743175ede 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct
> size_class *class,
>                 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.

> 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.

> 
> > 	-ss
> > 
> >> Anyway the compiler can do the same thing here without a WRITE_ONCE().
> > 
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