[PATCH] zram: avoid NULL pointer access in concurrent situation

[Weijie Yang <weijie.yang@samsung.com>]

There is a rare NULL pointer bug in mem_used_total_show() and
mem_used_max_store() in concurrent situation, like this:

zram is not initialized, process A is a mem_used_total reader which runs
periodically, while process B try to init zram.

	process A 				process B
access meta, get a NULL value
						init zram, done
init_done() is true
access meta->mem_pool, get a NULL pointer BUG

This patch fixes this issue.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 drivers/block/zram/zram_drv.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 0e63e8a..2ad0b5b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -99,11 +99,12 @@ static ssize_t mem_used_total_show(struct device *dev,
 {
 	u64 val = 0;
 	struct zram *zram = dev_to_zram(dev);
-	struct zram_meta *meta = zram->meta;
 
 	down_read(&zram->init_lock);
-	if (init_done(zram))
+	if (init_done(zram)) {
+		struct zram_meta *meta = zram->meta;
 		val = zs_get_total_pages(meta->mem_pool);
+	}
 	up_read(&zram->init_lock);
 
 	return scnprintf(buf, PAGE_SIZE, "%llu\n", val << PAGE_SHIFT);
@@ -173,16 +174,17 @@ static ssize_t mem_used_max_store(struct device *dev,
 	int err;
 	unsigned long val;
 	struct zram *zram = dev_to_zram(dev);
-	struct zram_meta *meta = zram->meta;
 
 	err = kstrtoul(buf, 10, &val);
 	if (err || val != 0)
 		return -EINVAL;
 
 	down_read(&zram->init_lock);
-	if (init_done(zram))
+	if (init_done(zram)) {
+		struct zram_meta *meta = zram->meta;
 		atomic_long_set(&zram->stats.max_used_pages,
 				zs_get_total_pages(meta->mem_pool));
+	}
 	up_read(&zram->init_lock);
 
 	return len;
-- 
1.7.0.4