dm snapshot: track snapshot reads

Whenever a snapshot read gets mapped through to the origin, track it in
a per-snapshot hash table indexed by chunk number, using memory allocated
from a new per-snapshot mempool.

We need to track these reads to avoid race conditions which will be fixed
by patches that follow.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/drivers/md/dm-snap.h b/drivers/md/dm-snap.h
index 24f9fb73b982..70dc961f40d8 100644
--- a/drivers/md/dm-snap.h
+++ b/drivers/md/dm-snap.h
@@ -130,6 +130,10 @@ struct exception_store {
         void *context;
 };
 
+#define DM_TRACKED_CHUNK_HASH_SIZE      16
+#define DM_TRACKED_CHUNK_HASH(x)        ((unsigned long)(x) & \
+                                         (DM_TRACKED_CHUNK_HASH_SIZE - 1))
+
 struct dm_snapshot {
         struct rw_semaphore lock;
         struct dm_target *ti;
@@ -174,6 +178,11 @@ struct dm_snapshot {
         /* Queue of snapshot writes for ksnapd to flush */
         struct bio_list queued_bios;
         struct work_struct queued_bios_work;
+
+        /* Chunks with outstanding reads */
+        mempool_t *tracked_chunk_pool;
+        spinlock_t tracked_chunk_lock;
+        struct hlist_head tracked_chunk_hash[DM_TRACKED_CHUNK_HASH_SIZE];
 };
 
 /*