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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] block: let blk_add/remove_aio_

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] block: let blk_add/remove_aio_context_notifier() tolerate BDS changes
Date: Fri, 9 Mar 2018 09:56:44 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 03/06/2018 02:48 PM, Stefan Hajnoczi wrote:
Commit 2019ba0a0197 ("block: Add AioContextNotifier functions to BB")
added blk_add/remove_aio_context_notifier() and implemented them by
passing through the bdrv_*() equivalent.

This doesn't work across bdrv_append(), which detaches child->bs and
re-attaches it to a new BlockDriverState.  When
blk_remove_aio_context_notifier() is called we will access the new BDS
instead of the one where the notifier was added!

From the point of view of the blk_*() API user, changes to the root BDS
should be transparent.

This patch maintains a list of AioContext notifiers in BlockBackend and
adds/removes them from the BlockDriverState as needed.

Reported-by: Stefano Panella <address@hidden>
Cc: Max Reitz <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
  block/block-backend.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++
  block/trace-events    |  2 ++
  2 files changed, 65 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index 94ffbb6a60..aa27698820 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -31,6 +31,13 @@
static AioContext *blk_aiocb_get_aio_context(BlockAIOCB *acb); +typedef struct BlockBackendAioNotifier {
+    void (*attached_aio_context)(AioContext *new_context, void *opaque);
+    void (*detach_aio_context)(void *opaque);

Why the difference in tense (past 'attached' vs. present 'detach')?

@@ -1827,12 +1877,25 @@ void blk_remove_aio_context_notifier(BlockBackend *blk,
                                       void (*detach_aio_context)(void *),
                                       void *opaque)
+    BlockBackendAioNotifier *notifier;
      BlockDriverState *bs = blk_bs(blk);
if (bs) {
          bdrv_remove_aio_context_notifier(bs, attached_aio_context,
                                           detach_aio_context, opaque);
+    QLIST_FOREACH(notifier, &blk->aio_notifiers, list) {
+        if (notifier->attached_aio_context == attached_aio_context &&
+            notifier->detach_aio_context == detach_aio_context &&
+            notifier->opaque == opaque) {
+            QLIST_REMOVE(notifier, list);

Don't you need to use QLIST_FOREACH_SAFE if you are going to modify the list during traversal?

Otherwise makes sense to me.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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