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

[Ganesh Mahendran <opensource.ganesh@gmail.com>]

Hello Sergey

2015-01-23 22:38 GMT+08:00 Sergey Senozhatsky <sergey.senozhatsky@gmail.com>:
> 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?

I changed from zram_attr_##name##_show to name##_show in commit:
fcf1bce zram: use DEVICE_ATTR_[RW|RO|WO] to define zram sys device attribute

I just want to keep the name consistent with others, like
disksize_show(), initstate_show().

Thanks.

>
>
> 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
>>
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