Re: [PATCH 2/2] zram: protect zram->stat race with init_lock

Sergey Senozhatsky
2015-01-23T14:38:19Z

On (01/23/15 14:58), Minchan Kim wrote:
> 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@kernel.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) \
> { \

a side note: I wasn't Cc'd in that patchset and found out it only when it's
been merged. I'm not sure I understand, why it has been renamed from specific
zram_X_show to X_show. what gives?

can't help, catches my eye every time, that rename has broken the original
formatting:

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 9250b3f..c567af5 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -44,7 +44,7 @@ static const char *default_compressor = "lzo";
static unsigned int num_devices = 1;

#define ZRAM_ATTR_RO(name) \
-static ssize_t name##_show(struct device *d, \
+static ssize_t name##_show(struct device *d, \
struct device_attribute *attr, char *b) \
{ \
struct zram *zram = dev_to_zram(d); \

I don't have any objections. but do we really want to wrap atomic ops in
semaphore? it is really such serious race?

-ss

> 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
>