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

[Sergey Senozhatsky]

Hello,

On (01/24/15 21:17), Ganesh Mahendran wrote:
> Hello Sergey
> 
> 2015-01-23 22:38 GMT+08:00 Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>:
> > 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@xxxxxxxxxx>
> >> ---
> >>
> >> 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().

aha, I see.

	-ss

> 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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