Re: [PATCH] zram: fix possible race when checking idle_strm

[Minchan Kim]

Hi Joonsoo,

On Fri, Aug 07, 2015 at 05:03:29PM +0900, Joonsoo Kim wrote:
> Currently, when we enter the wait state due to lack of idle stream,
> we check idle_strm list without holding the lock in expanding of
> wait_event define. In this case, some one can see stale value and
> process could fall into wait state without any upcoming wakeup process.
> Although I didn't see any error related to this race, it should be fixed.

Long time ago, I wondered about lost wake-up problem and found a article.

http://www.linuxjournal.com/article/8144

>From then, I have thought such issue shouldn't happen if something is
wrong since then and I believe it's same issue.

Could you point out exact code sequence about the problem you mentioned?

Thanks.

> 
> To fix it, we should check idle_strm with holding the lock and
> this patch implements it.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  drivers/block/zram/zcomp.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> index 80a62e7..837e9c3 100644
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -76,6 +76,17 @@ static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
>  	return zstrm;
>  }
>  
> +static bool avail_idle_strm(struct zcomp_strm_multi *zs)
> +{
> +	int avail;
> +
> +	spin_lock(&zs->strm_lock);
> +	avail = !list_empty(&zs->idle_strm);
> +	spin_unlock(&zs->strm_lock);
> +
> +	return avail;
> +}
> +
>  /*
>   * get idle zcomp_strm or wait until other process release
>   * (zcomp_strm_release()) one for us
> @@ -97,7 +108,7 @@ static struct zcomp_strm *zcomp_strm_multi_find(struct zcomp *comp)
>  		/* zstrm streams limit reached, wait for idle stream */
>  		if (zs->avail_strm >= zs->max_strm) {
>  			spin_unlock(&zs->strm_lock);
> -			wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));
> +			wait_event(zs->strm_wait, avail_idle_strm(zs));
>  			continue;
>  		}
>  		/* allocate new zstrm stream */
> @@ -109,7 +120,7 @@ static struct zcomp_strm *zcomp_strm_multi_find(struct zcomp *comp)
>  			spin_lock(&zs->strm_lock);
>  			zs->avail_strm--;
>  			spin_unlock(&zs->strm_lock);
> -			wait_event(zs->strm_wait, !list_empty(&zs->idle_strm));
> +			wait_event(zs->strm_wait, avail_idle_strm(zs));
>  			continue;
>  		}
>  		break;
> -- 
> 1.9.1
> 

-- 
Kind regards,
Minchan Kim
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