dm space map: disallow decrementing a reference count below zero

The old behaviour, returning -EINVAL if a ref_count of 0 would be
decremented, was removed in commit f722063 ("dm space map: optimise
sm_ll_dec and sm_ll_inc").  To fix this regression we return an error
code from the mutator function pointer passed to sm_ll_mutate() and have
dec_ref_count() return -EINVAL if the old ref_count is 0.

Add a DMERR to reflect the potential seriousness of this error.

Also, add missing dm_tm_unlock() to sm_ll_mutate()'s error path.

With this fix the following dmts regression test now passes:
 dmtest run --suite cache -n /metadata_use_kernel/

The next patch fixes the higher-level dm-array code that exposed this
regression.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.12+

1 files changed, 23 insertions, 9 deletions

diff --git a/drivers/md/persistent-data/dm-space-map-common.c b/drivers/md/persistent-data/dm-space-map-common.c
index 6058569fe86c..466a60bbd716 100644
--- a/drivers/md/persistent-data/dm-space-map-common.c
+++ b/drivers/md/persistent-data/dm-space-map-common.c
@@ -381,7 +381,7 @@ int sm_ll_find_free_block(struct ll_disk *ll, dm_block_t begin,
 }
 
 static int sm_ll_mutate(struct ll_disk *ll, dm_block_t b,
-                        uint32_t (*mutator)(void *context, uint32_t old),
+                        int (*mutator)(void *context, uint32_t old, uint32_t *new),
                         void *context, enum allocation_event *ev)
 {
         int r;
@@ -410,11 +410,17 @@ static int sm_ll_mutate(struct ll_disk *ll, dm_block_t b,
 
         if (old > 2) {
                 r = sm_ll_lookup_big_ref_count(ll, b, &old);
-                if (r < 0)
+                if (r < 0) {
+                        dm_tm_unlock(ll->tm, nb);
                         return r;
+                }
         }
 
-        ref_count = mutator(context, old);
+        r = mutator(context, old, &ref_count);
+        if (r) {
+                dm_tm_unlock(ll->tm, nb);
+                return r;
+        }
 
         if (ref_count <= 2) {
                 sm_set_bitmap(bm_le, bit, ref_count);
@@ -465,9 +471,10 @@ static int sm_ll_mutate(struct ll_disk *ll, dm_block_t b,
         return ll->save_ie(ll, index, &ie_disk);
 }
 
-static uint32_t set_ref_count(void *context, uint32_t old)
+static int set_ref_count(void *context, uint32_t old, uint32_t *new)
 {
-        return *((uint32_t *) context);
+        *new = *((uint32_t *) context);
+        return 0;
 }
 
 int sm_ll_insert(struct ll_disk *ll, dm_block_t b,
@@ -476,9 +483,10 @@ int sm_ll_insert(struct ll_disk *ll, dm_block_t b,
         return sm_ll_mutate(ll, b, set_ref_count, &ref_count, ev);
 }
 
-static uint32_t inc_ref_count(void *context, uint32_t old)
+static int inc_ref_count(void *context, uint32_t old, uint32_t *new)
 {
-        return old + 1;
+        *new = old + 1;
+        return 0;
 }
 
 int sm_ll_inc(struct ll_disk *ll, dm_block_t b, enum allocation_event *ev)
@@ -486,9 +494,15 @@ int sm_ll_inc(struct ll_disk *ll, dm_block_t b, enum allocation_event *ev)
         return sm_ll_mutate(ll, b, inc_ref_count, NULL, ev);
 }
 
-static uint32_t dec_ref_count(void *context, uint32_t old)
+static int dec_ref_count(void *context, uint32_t old, uint32_t *new)
 {
-        return old - 1;
+        if (!old) {
+                DMERR_LIMIT("unable to decrement a reference count below 0");
+                return -EINVAL;
+        }
+
+        *new = old - 1;
+        return 0;
 }
 
 int sm_ll_dec(struct ll_disk *ll, dm_block_t b, enum allocation_event *ev)