linux-kernel - [PATCH 2/2] zram: protect zram->stat race with init

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Date:	Fri, 23 Jan 2015 14:58:27 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Minchan Kim <minchan@...nel.org>
Subject: [PATCH 2/2] zram: protect zram->stat race with init_lock

The zram->stat handling should be procted by init_lock.
Otherwise, user could see stale value from the stat.

Signed-off-by: Minchan Kim <minchan@...nel.org>
---

I don't think it's stable material. The race is rare in real practice
and this stale stat value read is not a critical.

 drivers/block/zram/zram_drv.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 0299d82275e7..53f176f590b0 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -48,8 +48,13 @@ static ssize_t name##_show(struct device *d,		\
 				struct device_attribute *attr, char *b)	\
 {									\
 	struct zram *zram = dev_to_zram(d);				\
-	return scnprintf(b, PAGE_SIZE, "%llu\n",			\
-		(u64)atomic64_read(&zram->stats.name));			\
+	u64 val = 0;							\
+									\
+	down_read(&zram->init_lock);					\
+	if (init_done(zram))						\
+		val = atomic64_read(&zram->stats.name);			\
+	up_read(&zram->init_lock);					\
+	return scnprintf(b, PAGE_SIZE, "%llu\n", val);			\
 }									\
 static DEVICE_ATTR_RO(name);
 
@@ -67,8 +72,14 @@ static ssize_t disksize_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
 	struct zram *zram = dev_to_zram(dev);
+	u64 val = 0;
+
+	down_read(&zram->init_lock);
+	if (init_done(zram))
+		val = zram->disksize;
+	up_read(&zram->init_lock);
 
-	return scnprintf(buf, PAGE_SIZE, "%llu\n", zram->disksize);
+	return scnprintf(buf, PAGE_SIZE, "%llu\n", val);
 }
 
 static ssize_t initstate_show(struct device *dev,
@@ -88,9 +99,14 @@ static ssize_t orig_data_size_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
 	struct zram *zram = dev_to_zram(dev);
+	u64 val = 0;
+
+	down_read(&zram->init_lock);
+	if (init_done(zram))
+		val = atomic64_read(&zram->stats.pages_stored) << PAGE_SHIFT;
+	up_read(&zram->init_lock);
 
-	return scnprintf(buf, PAGE_SIZE, "%llu\n",
-		(u64)(atomic64_read(&zram->stats.pages_stored)) << PAGE_SHIFT);
+	return scnprintf(buf, PAGE_SIZE, "%llu\n", val);
 }
 
 static ssize_t mem_used_total_show(struct device *dev,
@@ -957,10 +973,6 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
 	struct bio_vec bv;
 
 	zram = bdev->bd_disk->private_data;
-	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
-		atomic64_inc(&zram->stats.invalid_io);
-		return -EINVAL;
-	}
 
 	down_read(&zram->init_lock);
 	if (unlikely(!init_done(zram))) {
@@ -968,6 +980,13 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
 		goto out_unlock;
 	}
 
+	if (!valid_io_request(zram, sector, PAGE_SIZE)) {
+		atomic64_inc(&zram->stats.invalid_io);
+		err = -EINVAL;
+		goto out_unlock;
+	}
+
+
 	index = sector >> SECTORS_PER_PAGE_SHIFT;
 	offset = sector & (SECTORS_PER_PAGE - 1) << SECTOR_SHIFT;
 
-- 
1.9.1

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