Ack. Thanks.
On Fri, Sep 05, 2014 at 03:11:28AM +0300, Anssi Hannula wrote:
> When a writeback or a promotion of a block is completed, the cell of
> that block is removed from the prison, the block is marked as clean, and
> the clear_dirty() callback of the cache policy is called.
>
> Unfortunately, performing those actions in this order allows an incoming
> new write bio for that block to come in before clearing the dirty status
> is completed and therefore possibly causing one of these two scenarios:
>
> Scenario A:
>
> Thread 1 Thread 2
> cell_defer() .
> - cell removed from prison .
> - detained bios queued .
> . incoming write bio
> . remapped to cache
> . set_dirty() called,
> . but block already dirty
> . => it does nothing
> clear_dirty() .
> - block marked clean .
> - policy clear_dirty() called .
>
> Result: Block is marked clean even though it is actually dirty. No
> writeback will occur.
>
> Scenario B:
>
> Thread 1 Thread 2
> cell_defer() .
> - cell removed from prison .
> - detained bios queued .
> clear_dirty() .
> - block marked clean .
> . incoming write bio
> . remapped to cache
> . set_dirty() called
> . - block marked dirty
> . - policy set_dirty() called
> - policy clear_dirty() called .
>
> Result: Block is properly marked as dirty, but policy thinks it is clean
> and therefore never asks us to writeback it.
> This case is visible in "dmsetup status" dirty block count (which
> normally decreases to 0 on a quiet device).
>
> Fix these issues by calling clear_dirty() before calling cell_defer().
> Incoming bios for that block will then be detained in the cell and
> released only after clear_dirty() has completed, so the race will not
> occur.
>
> Found by inspecting the code after noticing spurious dirty counts
> (scenario B).
>
> Signed-off-by: Anssi Hannula <anssi.hannula@xxxxxx>
> Cc: Joe Thornber <ejt@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>
> > Unfortunately it seems there is some other potentially more serious bug
> > still in there...
>
> After looking through the code that indeed seems to be the case, as
> explained above.
>
> Unless I'm missing something?
>
> I can't say with 100% certainty if this fixes the spurious counts I saw
> since those took quite a long time (1-2 weeks?) to appear and the load
> of that system is somewhat irregular.
>
>
> drivers/md/dm-cache-target.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
> index 1af40ee209e2..7130505c2425 100644
> --- a/drivers/md/dm-cache-target.c
> +++ b/drivers/md/dm-cache-target.c
> @@ -895,8 +895,8 @@ static void migration_success_pre_commit(struct dm_cache_migration *mg)
> struct cache *cache = mg->cache;
>
> if (mg->writeback) {
> - cell_defer(cache, mg->old_ocell, false);
> clear_dirty(cache, mg->old_oblock, mg->cblock);
> + cell_defer(cache, mg->old_ocell, false);
> cleanup_migration(mg);
> return;
>
> @@ -951,13 +951,13 @@ static void migration_success_post_commit(struct dm_cache_migration *mg)
> }
>
> } else {
> + clear_dirty(cache, mg->new_oblock, mg->cblock);
> if (mg->requeue_holder)
> cell_defer(cache, mg->new_ocell, true);
> else {
> bio_endio(mg->new_ocell->holder, 0);
> cell_defer(cache, mg->new_ocell, false);
> }
> - clear_dirty(cache, mg->new_oblock, mg->cblock);
> cleanup_migration(mg);
> }
> }
> --
> 1.8.4.5
>
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