[Qemu-devel] [PATCH v4 1/4] block/io: fix copy_range

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Here two things are fixed:

1. Architecture

On each recursion step, we go to the child of src or dst, only for one
of them. So, it's wrong to create tracked requests for both on each
step. It leads to tracked requests duplication.

2. Wait for serializing requests on write path independently of
   BDRV_REQ_NO_SERIALISING

Before commit 9ded4a01149 "backup: Use copy offloading",
BDRV_REQ_NO_SERIALISING was used for only one case: read in
copy-on-write operation during backup. Also, the flag was handled only
on read path (in bdrv_co_preadv and bdrv_aligned_preadv).

After 9ded4a01149, flag is used for not waiting serializing operations
on backup target (in same case of copy-on-write operation). This
behavior change is unsubstantiated and potentially dangerous, let's
drop it and add additional asserts and documentation.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block.h |  13 +++++++
 block/io.c            | 103 +++++++++++++++++++++++++++++++-------------------
 2 files changed, 78 insertions(+), 38 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index e5c7759a0c..a06a4d27de 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -50,6 +50,19 @@ typedef enum {
      * opened with BDRV_O_UNMAP.
      */
     BDRV_REQ_MAY_UNMAP          = 0x4,
+
+    /* The BDRV_REQ_NO_SERIALISING means that we don't want to
+     * wait_serialising_requests(), when reading.
+     *
+     * This flag is used for backup copy on write operation, when we need to
+     * read old data before write (write notifier triggered). It is ok, due to
+     * we already waited for serializing requests in initiative write (see
+     * bdrv_aligned_pwritev), and it is necessary for the case when initiative
+     * write is serializing itself (we'll dead lock waiting it).
+     *
+     * The described case is the only usage for the flag for now, so, it is
+     * supported only for read operation and restricted for write.
+     */
     BDRV_REQ_NO_SERIALISING     = 0x8,
     BDRV_REQ_FUA                = 0x10,
     BDRV_REQ_WRITE_COMPRESSED   = 0x20,
diff --git a/block/io.c b/block/io.c
index 1a2272fad3..621b21c455 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1572,6 +1572,8 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child,
     max_transfer = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_transfer, INT_MAX),
                                    align);
 
+    /* BDRV_REQ_NO_SERIALISING is only for read operation */
+    assert(!(flags & BDRV_REQ_NO_SERIALISING));
     waited = wait_serialising_requests(req);
     assert(!waited || !req->serialising);
     assert(req->overlap_offset <= offset);
@@ -2888,15 +2890,19 @@ void bdrv_unregister_buf(BlockDriverState *bs, void *host)
     }
 }
 
-static int coroutine_fn bdrv_co_copy_range_internal(BdrvChild *src,
-                                                    uint64_t src_offset,
-                                                    BdrvChild *dst,
-                                                    uint64_t dst_offset,
-                                                    uint64_t bytes,
-                                                    BdrvRequestFlags flags,
-                                                    bool recurse_src)
+/* Common part of bdrv_co_copy_range_from and bdrv_co_copy_range_to.
+ *
+ * Return -errno on failure,
+ *        0 if successfully handled by bdrv_co_pwrite_zeroes
+ *        1 to continue copy_range operation
+ */
+static int coroutine_fn bdrv_co_copy_range_check(BdrvChild *src,
+                                                 uint64_t src_offset,
+                                                 BdrvChild *dst,
+                                                 uint64_t dst_offset,
+                                                 uint64_t bytes,
+                                                 BdrvRequestFlags flags)
 {
-    BdrvTrackedRequest src_req, dst_req;
     int ret;
 
     if (!dst || !dst->bs) {
@@ -2923,33 +2929,8 @@ static int coroutine_fn bdrv_co_copy_range_internal(BdrvChild *src,
         || src->bs->encrypted || dst->bs->encrypted) {
         return -ENOTSUP;
     }
-    bdrv_inc_in_flight(src->bs);
-    bdrv_inc_in_flight(dst->bs);
-    tracked_request_begin(&src_req, src->bs, src_offset,
-                          bytes, BDRV_TRACKED_READ);
-    tracked_request_begin(&dst_req, dst->bs, dst_offset,
-                          bytes, BDRV_TRACKED_WRITE);
 
-    if (!(flags & BDRV_REQ_NO_SERIALISING)) {
-        wait_serialising_requests(&src_req);
-        wait_serialising_requests(&dst_req);
-    }
-    if (recurse_src) {
-        ret = src->bs->drv->bdrv_co_copy_range_from(src->bs,
-                                                    src, src_offset,
-                                                    dst, dst_offset,
-                                                    bytes, flags);
-    } else {
-        ret = dst->bs->drv->bdrv_co_copy_range_to(dst->bs,
-                                                  src, src_offset,
-                                                  dst, dst_offset,
-                                                  bytes, flags);
-    }
-    tracked_request_end(&src_req);
-    tracked_request_end(&dst_req);
-    bdrv_dec_in_flight(src->bs);
-    bdrv_dec_in_flight(dst->bs);
-    return ret;
+    return 1;
 }
 
 /* Copy range from @src to @dst.
@@ -2960,8 +2941,31 @@ int coroutine_fn bdrv_co_copy_range_from(BdrvChild *src, uint64_t src_offset,
                                          BdrvChild *dst, uint64_t dst_offset,
                                          uint64_t bytes, BdrvRequestFlags flags)
 {
-    return bdrv_co_copy_range_internal(src, src_offset, dst, dst_offset,
-                                       bytes, flags, true);
+    BdrvTrackedRequest req;
+    int ret;
+
+    ret = bdrv_co_copy_range_check(src, src_offset, dst, dst_offset, bytes,
+                                   flags);
+    if (ret <= 0) {
+        return ret;
+    }
+
+    bdrv_inc_in_flight(src->bs);
+    tracked_request_begin(&req, src->bs, src_offset, bytes, BDRV_TRACKED_READ);
+
+    if (!(flags & BDRV_REQ_NO_SERIALISING)) {
+        wait_serialising_requests(&req);
+    }
+
+    ret = src->bs->drv->bdrv_co_copy_range_from(src->bs,
+                                                src, src_offset,
+                                                dst, dst_offset,
+                                                bytes, flags);
+
+    tracked_request_end(&req);
+    bdrv_dec_in_flight(src->bs);
+
+    return ret;
 }
 
 /* Copy range from @src to @dst.
@@ -2972,8 +2976,31 @@ int coroutine_fn bdrv_co_copy_range_to(BdrvChild *src, uint64_t src_offset,
                                        BdrvChild *dst, uint64_t dst_offset,
                                        uint64_t bytes, BdrvRequestFlags flags)
 {
-    return bdrv_co_copy_range_internal(src, src_offset, dst, dst_offset,
-                                       bytes, flags, false);
+    BdrvTrackedRequest req;
+    int ret;
+
+    ret = bdrv_co_copy_range_check(src, src_offset, dst, dst_offset, bytes,
+                                   flags);
+    if (ret <= 0) {
+        return ret;
+    }
+
+    bdrv_inc_in_flight(dst->bs);
+    tracked_request_begin(&req, dst->bs, dst_offset, bytes, BDRV_TRACKED_WRITE);
+
+    /* BDRV_REQ_NO_SERIALISING is only for read operation, so we ignore it in
+     * flags. */
+    wait_serialising_requests(&req);
+
+    ret = dst->bs->drv->bdrv_co_copy_range_to(dst->bs,
+                                              src, src_offset,
+                                              dst, dst_offset,
+                                              bytes, flags);
+
+    tracked_request_end(&req);
+    bdrv_dec_in_flight(dst->bs);
+
+    return ret;
 }
 
 int coroutine_fn bdrv_co_copy_range(BdrvChild *src, uint64_t src_offset,
-- 
2.11.1