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

Sergey Senozhatsky
2015-08-07T09:36:46Z

On (08/07/15 17:03), 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.
>
> To fix it, we should check idle_strm with holding the lock and
> this patch implements it.
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

> ---
> 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;
> --