[3.13.y.z extended stable] Patch "dm cache: fix race causing dirty blocks to be marked as clean" has been added to staging queue

Kamal Mostafa kamal at canonical.com
Wed Oct 8 22:14:09 UTC 2014


This is a note to let you know that I have just added a patch titled

    dm cache: fix race causing dirty blocks to be marked as clean

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.9.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 2cf00e2624624b90055065ae00a04874f54bdcb1 Mon Sep 17 00:00:00 2001
From: Anssi Hannula <anssi.hannula at iki.fi>
Date: Fri, 5 Sep 2014 03:11:28 +0300
Subject: 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 <anssi.hannula at iki.fi>
Acked-by: Joe Thornber <ejt at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 ae73ee5..99474ef 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -881,8 +881,8 @@ 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;

@@ -937,13 +937,13 @@ 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);
 	}
 }
--
1.9.1





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