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List:       dm-devel
Subject:    [dm-devel] dm-cache race on nr_dirty in set_dirty/clear_dirty?
From:       Anssi Hannula <anssi.hannula () iki ! fi>
Date:       2014-07-26 4:07:53
Message-ID: 53D32999.6080206 () iki ! fi
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Hi,

I'm seeing the following "dmsetup status" on one volume:
> 0 5368709120 cache 8 3926/32768 128 335265/1978880 5589425 8052258 2254781 3910141 \
> 0 335265 4294967293 1 writeback 2 migration_threshold 2048 mq 10 random_threshold 4 \
> sequential_threshold 512 discard_promote_adjustment 1 read_promote_adjustment 4 \
> write_promote_adjustment 8

Note the clearly wrong 4294967293 in the nr_dirty field.

Looking at the code, I see nr_dirty is set in the following functions in
dm-cache-target.c:

> 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);
> policy_set_dirty(cache->policy, oblock);
> }
> }
> 
> static void clear_dirty(struct cache *cache, dm_oblock_t oblock, dm_cblock_t \
> cblock) {
> 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))
> dm_table_event(cache->ti->table);
> }
> }

That looks like a race to me? As nothing is protecting cache->nr_dirty
from multiple access (unlike cache->dirty_bitset).

Unless I'm missing something, as I'm not familiar with this code...

-- 
Anssi Hannula

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