From 6f0e09c86dea00dd6b9a346e8ea62a97794bf80c Mon Sep 17 00:00:00 2001 From: Anssi Hannula Date: Fri, 5 Sep 2014 03:11:28 +0300 Subject: [PATCH] dm cache: fix race causing dirty blocks to be marked as clean commit 40aa978eccec61347cd47b97c598df49acde8be5 upstream. 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 Acked-by: Joe Thornber Signed-off-by: Mike Snitzer Signed-off-by: Greg Kroah-Hartman --- 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 735e939a846d..2331543005b2 100644 --- a/drivers/md/dm-cache-target.c +++ b/drivers/md/dm-cache-target.c @@ -890,8 +890,8 @@ 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; @@ -946,13 +946,13 @@ 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); } } -- 2.39.2