On 07/06/2018 04:32 PM, Eric Blake wrote:
On 07/05/2018 02:46 AM, Vladimir Sementsov-Ogievskiy wrote:
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.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
include/block/block.h | 13 +++++++++++++
block/io.c | 7 ++++++-
2 files changed, 19 insertions(+), 1 deletion(-)
Commenting only on the grammar:
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.
Either:
/* BDRV_REQ_NO_SERALISING means that...
or
/* The BDRV_REQ_NO_SERIALISING flag means that...
s/want to/want/
Or, after reading patch 3/4,
BDRV_REQ_NO_SERAILISING bypasses request serialisation during reads.
(where the counterpart starts:
BDRV_REQ_SERIALISING forces request serialisation during writes.
)
+ *
+ * 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).
It is okay since we already waited for other serializing requests in the
initiating write (see bdrv_aligned_pwritev), and it is necessary since
the initiating write is already serializing (without the flag, the read
would deadlock waiting for the write to complete).
+ *
+ * The described case is the only usage for the flag for now, so,
it is
+ * supported only for read operation and restricted for write.
This last sentence is rather wordy; I'm fine with just:
The flag is only valid during read operations.
Or even just drop this last paragraph, since the first sentence of the
comment already stated it was only for reads.
+ */
BDRV_REQ_NO_SERIALISING = 0x8,
BDRV_REQ_FUA = 0x10,
BDRV_REQ_WRITE_COMPRESSED = 0x20,
We're inconsistent on which flags we document; it might be nice to have
a comment for each of them. But not necessarily this patch's problem.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
