rbd: access snapshot context and mapping size safely

These fields may both change while the image is mapped if a snapshot
is created or deleted or the image is resized.  They are guarded by
rbd_dev->header_rwsem, so hold that while reading them, and store a
local copy to refer to outside of the critical section. The local copy
will stay consistent since the snapshot context is reference counted,
and the mapping size is just a u64. This prevents torn loads from
giving us inconsistent values.

Move reading header.snapc into the caller of rbd_img_request_create()
so that we only need to take the semaphore once. The read-only caller,
rbd_parent_request_create() can just pass NULL for snapc, since the
snapshot context is only relevant for writes.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>

1 files changed, 21 insertions, 13 deletions

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index ce457db5d847..eea44ce2d537 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -2057,7 +2057,8 @@ static bool rbd_dev_parent_get(struct rbd_device *rbd_dev)
 static struct rbd_img_request *rbd_img_request_create(
                                         struct rbd_device *rbd_dev,
                                         u64 offset, u64 length,
-                                        bool write_request)
+                                        bool write_request,
+                                        struct ceph_snap_context *snapc)
 {
         struct rbd_img_request *img_request;
 
@@ -2065,12 +2066,6 @@ static struct rbd_img_request *rbd_img_request_create(
         if (!img_request)
                 return NULL;
 
-        if (write_request) {
-                down_read(&rbd_dev->header_rwsem);
-                ceph_get_snap_context(rbd_dev->header.snapc);
-                up_read(&rbd_dev->header_rwsem);
-        }
-
         img_request->rq = NULL;
         img_request->rbd_dev = rbd_dev;
         img_request->offset = offset;
@@ -2078,7 +2073,7 @@ static struct rbd_img_request *rbd_img_request_create(
         img_request->flags = 0;
         if (write_request) {
                 img_request_write_set(img_request);
-                img_request->snapc = rbd_dev->header.snapc;
+                img_request->snapc = snapc;
         } else {
                 img_request->snap_id = rbd_dev->spec->snap_id;
         }
@@ -2134,8 +2129,8 @@ static struct rbd_img_request *rbd_parent_request_create(
         rbd_assert(obj_request->img_request);
         rbd_dev = obj_request->img_request->rbd_dev;
 
-        parent_request = rbd_img_request_create(rbd_dev->parent,
-                                                img_offset, length, false);
+        parent_request = rbd_img_request_create(rbd_dev->parent, img_offset,
+                                                length, false, NULL);
         if (!parent_request)
                 return NULL;
 
@@ -3183,9 +3178,11 @@ out:
 static void rbd_handle_request(struct rbd_device *rbd_dev, struct request *rq)
 {
         struct rbd_img_request *img_request;
+        struct ceph_snap_context *snapc = NULL;
         u64 offset = (u64)blk_rq_pos(rq) << SECTOR_SHIFT;
         u64 length = blk_rq_bytes(rq);
         bool wr = rq_data_dir(rq) == WRITE;
+        u64 mapping_size;
         int result;
 
         /* Ignore/skip any zero-length requests */
@@ -3226,14 +3223,23 @@ static void rbd_handle_request(struct rbd_device *rbd_dev, struct request *rq)
                 goto err_rq;    /* Shouldn't happen */
         }
 
-        if (offset + length > rbd_dev->mapping.size) {
+        down_read(&rbd_dev->header_rwsem);
+        mapping_size = rbd_dev->mapping.size;
+        if (wr) {
+                snapc = rbd_dev->header.snapc;
+                ceph_get_snap_context(snapc);
+        }
+        up_read(&rbd_dev->header_rwsem);
+
+        if (offset + length > mapping_size) {
                 rbd_warn(rbd_dev, "beyond EOD (%llu~%llu > %llu)", offset,
-                         length, rbd_dev->mapping.size);
+                         length, mapping_size);
                 result = -EIO;
                 goto err_rq;
         }
 
-        img_request = rbd_img_request_create(rbd_dev, offset, length, wr);
+        img_request = rbd_img_request_create(rbd_dev, offset, length, wr,
+                                             snapc);
         if (!img_request) {
                 result = -ENOMEM;
                 goto err_rq;
@@ -3256,6 +3262,8 @@ err_rq:
         if (result)
                 rbd_warn(rbd_dev, "%s %llx at %llx result %d",
                          wr ? "write" : "read", length, offset, result);
+        if (snapc)
+                ceph_put_snap_context(snapc);
         blk_end_request_all(rq, result);
 }