linux-kernel - [09/85] zram: use zram->lock to protect zram_free

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Date:	Wed, 24 Jul 2013 15:02:45 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Jiang Liu" <jiang.liu@...wei.com>, "Jiang Liu" <liuj97@...il.com>
Subject: [09/85] zram: use zram->lock to protect zram_free_page() in swap
 free notify path

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

------------------

From: Jiang Liu <liuj97@...il.com>

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@...wei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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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