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Re: [Qemu-devel] [PATCH v3 1/4] block: disallow BDRV_REQ_NO_SERIALISING

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 1/4] block: disallow BDRV_REQ_NO_SERIALISING for write
Date: Fri, 6 Jul 2018 16:46:38 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

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 <address@hidden>
  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.


/* BDRV_REQ_NO_SERALISING means that...


/* 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,

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

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