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

commit 57ab048532c0d975538cebd4456491b5c34248f4 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
index e87af29..785a947 100644 (file)
--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -702,7 +702,9 @@ 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);
 }
 

diff --git a/drivers/staging/zram/zram_drv.h b/drivers/staging/zram/zram_drv.h
index e5cd246..87f2fec 100644 (file)
--- a/drivers/staging/zram/zram_drv.h
+++ b/drivers/staging/zram/zram_drv.h
@@ -107,8 +107,9 @@ struct zram {
        void *compress_buffer;
        struct table *table;
        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;