linux-kernel - Re: [PATCH] dm cache: fix race affecting dirty block count

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Message-ID: <CAJhHMCCFz5qpxocMbF1POBbFs6zPVaoM_j-52bw5vwpm1MV9aw@mail.gmail.com>
Date:	Sat, 2 Aug 2014 22:10:18 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	anssi.hannula@....fi
Cc:	ejt@...hat.com, snitzer@...hat.com,
	LKML <linux-kernel@...r.kernel.org>,
	torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] dm cache: fix race affecting dirty block count

Corrently adding Anssi this time.

On Sat, Aug 2, 2014 at 10:00 PM, Pranith Kumar <bobby.prani@...il.com> wrote:
> Hello Anssi, Joe, Mike,
>
> I just found your patch in the latest rc and wanted to ask a few
> questions. I am not sure how this patch helps at all other than luck in
> that dm_cblock_t type is of type int32_t, which should guarantee that it
> is atomic on most platforms. Which begs the question, what platform did
> you encounter this problem?
>
> The patch purports to solve a race condition by making use of atomic_t.
> I am not sure that is enough. If indeed there is a race you need to use
> smp_mb__{before/after}_atomic() for both your uses of atomic_inc() and
> atomic_set().
>
> Also I have a concern about why this mail was not CC'ed on LKML. I had
> to go to some difficulty in finding this patch. So please CC LKML for
> such patches.
>
> Thanks,
> --
> Pranith
>
> -- Begin forwarded Message --
>
>
> nr_dirty is updated without locking, causing it to drift so that it is
> non-zero (either a small positive integer, or a very large one when an
> underflow occurs) even when there are no actual dirty blocks.
>
> Fix that by using an atomic type for nr_dirty.
>
> Signed-off-by: Anssi Hannula <anssi hannula iki fi>
> Cc: Joe Thornber <ejt redhat com>
> Cc: stable vger kernel org
>
> ---
>
> So here's a patch for the issue I reported a few days ago.
>
> I'm not sure what the actual effects of having wrong nr_dirty are
> beyond the wrong value reported to userspace. Having wrong nr_dirty
> causes dm_table_event(cache->ti->table) to be called at incorrect times,
> but I'm not sure what the implications of that are.
>
> So someone more knowledged should really amend my commit message
> accordingly so that it explains the user-visible issues :)
>
> I did notice that after a reboot (after having incorrect nr_dirty) the
> entire cache was considered to be dirty and a full writeback occurred,
> but I don't know if that was related at all.
>
>
>  drivers/md/dm-cache-target.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
> index 0df3ec085ebb..83a7eb5ef113 100644
> --- a/drivers/md/dm-cache-target.c
> +++ b/drivers/md/dm-cache-target.c
> @@ -154,7 +154,7 @@ struct cache {
>         /*
>          * cache_size entries, dirty if set
>          */
> -       dm_cblock_t nr_dirty;
> +       atomic_t nr_dirty;
>         unsigned long *dirty_bitset;
>
>         /*
> @@ -403,7 +403,7 @@ static bool is_dirty(struct cache *cache, dm_cblock_t b)
>  static void set_dirty(struct cache *cache, dm_oblock_t oblock, dm_cblock_t cblock)
>  {
>         if (!test_and_set_bit(from_cblock(cblock), cache->dirty_bitset)) {
> -               cache->nr_dirty = to_cblock(from_cblock(cache->nr_dirty) + 1);
> +               atomic_inc(&cache->nr_dirty);
>                 policy_set_dirty(cache->policy, oblock);
>         }
>  }
> @@ -412,8 +412,7 @@ static void clear_dirty(struct cache *cache, dm_oblock_t oblock, dm_cblock_t cbl
>  {
>         if (test_and_clear_bit(from_cblock(cblock), cache->dirty_bitset)) {
>                 policy_clear_dirty(cache->policy, oblock);
> -               cache->nr_dirty = to_cblock(from_cblock(cache->nr_dirty) - 1);
> -               if (!from_cblock(cache->nr_dirty))
> +               if (atomic_dec_return(&cache->nr_dirty) == 0)
>                         dm_table_event(cache->ti->table);
>         }
>  }
> @@ -2003,7 +2002,7 @@ static int cache_create(struct cache_args *ca, struct cache **result)
>         init_waitqueue_head(&cache->migration_wait);
>
>         r = -ENOMEM;
> -       cache->nr_dirty = 0;
> +       atomic_set(&cache->nr_dirty, 0);
>         cache->dirty_bitset = alloc_bitset(from_cblock(cache->cache_size));
>         if (!cache->dirty_bitset) {
>                 *error = "could not allocate dirty bitset";
> @@ -2513,7 +2512,7 @@ static void cache_status(struct dm_target *ti, status_type_t type,
>                        (unsigned) atomic_read(&cache->stats.demotion),
>                        (unsigned) atomic_read(&cache->stats.promotion),
>                        (unsigned long long) from_cblock(residency),
> -                      cache->nr_dirty);
> +                      (unsigned) atomic_read(&cache->nr_dirty));
>
>                 if (cache->features.write_through)
>                         DMEMIT("1 writethrough ");
> --
> 1.8.4.5
>
> -- End forwarded Message --
>



-- 
Pranith
--
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