Hi Junil,
On Fri, Jan 15, 2016 at 09:36:24AM +0900, Junil Lee wrote:
> To prevent unlock at the not correct situation, tagging the new obj to
> assure lock in migrate_zspage() before right unlock path.
>
> Two functions are in race condition by tag which set 1 on last bit of
> obj, however unlock succrently when update new obj to handle before call
> unpin_tag() which is right unlock path.
>
> summarize this problem by call flow as below:
>
> CPU0 CPU1
> migrate_zspage
> find_alloced_obj()
> trypin_tag() -- obj |= HANDLE_PIN_BIT
> obj_malloc() -- new obj is not set zs_free
> record_obj() -- unlock and break sync pin_tag() -- get lock
> unpin_tag()
It's really good catch!
I think it should be stable material. For that, we should know this
patch fixes what kinds of problem.
What do you see problem? I mean please write down the oops you saw and
verify that the patch fixes your problem. :)
Minor nit below
>
> Signed-off-by: Junil Lee <junil0814.lee@xxxxxxx>
> ---
> mm/zsmalloc.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index e7414ce..bb459ef 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1635,6 +1635,7 @@ 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++;
> + free_obj |= BIT(HANDLE_PIN_BIT);
> record_obj(handle, free_obj);
I think record_obj should store free_obj to *handle with masking off least bit.
IOW, how about this?
record_obj(handle, obj)
{
*(unsigned long)handle = obj & ~(1<<HANDLE_PIN_BIT);
}
Thanks a lot!
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