[09/85] zram: use zram->lock to protect zram_free_page() in swapfree notify path

From: Ben Hutchings
Date: Wed Jul 24 2013 - 10:35:20 EST

3.2.49-rc1 review patch. If anyone has any objections, please let me know.


From: Jiang Liu <liuj97@xxxxxxxxx>

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

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@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
drivers/staging/zram/zram_drv.c | 2 ++
drivers/staging/zram/zram_drv.h | 5 +++--
2 files changed, 5 insertions(+), 2 deletions(-)

--- 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
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);

--- 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;

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