Re: [PATCH 1/2] zram: free meta out of init_lock

Sergey Senozhatsky
2015-01-23T14:24:05Z

On (01/23/15 14:58), Minchan Kim wrote:
> We don't need to call zram_meta_free, zcomp_destroy and zs_free
> under init_lock. What we need to prevent race with init_lock
> in reset is setting NULL into zram->meta (ie, init_done).
> This patch does it.
>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> drivers/block/zram/zram_drv.c | 28 ++++++++++++++++------------
> 1 file changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 9250b3f54a8f..0299d82275e7 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -708,6 +708,7 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> {
> size_t index;
> struct zram_meta *meta;
> + struct zcomp *comp;
>
> down_write(&zram->init_lock);
>
> @@ -719,20 +720,10 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> }
>
> meta = zram->meta;
> - /* Free all pages that are still in this zram device */
> - for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> - unsigned long handle = meta->table[index].handle;
> - if (!handle)
> - continue;
> -
> - zs_free(meta->mem_pool, handle);
> - }
> -
> - zcomp_destroy(zram->comp);

I'm not so sure about moving zcomp destruction. if we would have detached it
from zram, then yes. otherwise, think of zram ->destoy vs ->init race.

suppose,
CPU1 waits for down_write() init lock in disksize_store() with new comp already allocated;
CPU0 detaches ->meta and releases write init lock;
CPU1 grabs the lock and does zram->comp = comp;
CPU0 reaches the point of zcomp_destroy(zram->comp);

I'd probably prefer to keep zcomp destruction on its current place. I
see a little real value in introducing zcomp detaching and moving
destruction out of init_lock.

-ss

> + comp = zram->comp;
> + zram->meta = NULL;
> zram->max_comp_streams = 1;
>
> - zram_meta_free(zram->meta);
> - zram->meta = NULL;
> /* Reset stats */
> memset(&zram->stats, 0, sizeof(zram->stats));
>
> @@ -742,6 +733,19 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
>
> up_write(&zram->init_lock);
>
> + /* Free all pages that are still in this zram device */
> + for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> + unsigned long handle = meta->table[index].handle;
> +
> + if (!handle)
> + continue;
> +
> + zs_free(meta->mem_pool, handle);
> + }
> +
> + zcomp_destroy(comp);
> + zram_meta_free(meta);
> +
> /*
> * Revalidate disk out of the init_lock to avoid lockdep splat.
> * It's okay because disk's capacity is protected by init_lock
> --
> 1.9.1
>