zram: use zram->lock to protect zram_free_page() in swap free notify path

zram_slot_free_notify() is free-running without any protection from
concurrent operations. So there are race conditions between
zram_bvec_read()/zram_bvec_write() and zram_slot_free_notify(),
and possible consequences include:
1) Trigger BUG_ON(!handle) on zram_bvec_write() side.
2) Access to freed pages on zram_bvec_read() side.
3) Break some fields (bad_compress, good_compress, pages_stored)
   in zram->stats if the swap layer makes concurrently call to
   zram_slot_free_notify().

So enhance zram_slot_free_notify() to acquire writer lock on zram->lock
before calling zram_free_page().
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zram/zram_drv.c

@@ -582,7 +582,9 @@ static void zram_slot_free_notify(struct block_device *bdev,
 	struct zram *zram;
 
 	zram = bdev->bd_disk->private_data;
+	down_write(&zram->lock);
 	zram_free_page(zram, index);
+	up_write(&zram->lock);
 	zram_stat64_inc(zram, &zram->stats.notify_free);
 }
 

drivers/staging/zram/zram_drv.h

@@ -93,8 +93,9 @@ struct zram_meta {
 struct zram {
 	struct zram_meta *meta;
 	spinlock_t stat64_lock;	/* protect 64-bit stats */
-	struct rw_semaphore lock; /* protect compression buffers and table
-				   * against concurrent read and writes */
+	struct rw_semaphore lock; /* protect compression buffers, table,
+				   * 32bit stat counters against concurrent
+				   * notifications, reads and writes */
 	struct request_queue *queue;
 	struct gendisk *disk;
 	int init_done;