[Qemu-devel] [PATCH v6 14/15] block: Rewrite bdrv_close_all()

[Max Reitz <mreitz@redhat.com>]

This patch rewrites bdrv_close_all(): Until now, all root BDSs have been
force-closed. This is bad because it can lead to cached data not being
flushed to disk.

Instead, try to make all reference holders relinquish their reference
voluntarily:

1. All BlockBackend users are handled by making all BBs simply eject
   their BDS tree. Since a BDS can never be on top of a BB, this will
   not cause any of the issues as seen with the force-closing of BDSs.
   The references will be relinquished and any further access to the BB
   will fail gracefully.
2. All BDSs which are owned by the monitor itself (because they do not
   have a BB) are relinquished next.
3. Besides BBs and the monitor, block jobs and other BDSs are the only
   things left that can hold a reference to BDSs. After every remaining
   block job has been canceled, there should not be any BDSs left (and
   the loop added here will always terminate (as long as NDEBUG is not
   defined), because either all_bdrv_states will be empty or there will
   not be any block job left to cancel, failing the assertion).

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block.c | 45 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/block.c b/block.c
index cf5dadc..95a7f98 100644
--- a/block.c
+++ b/block.c
@@ -1905,9 +1905,7 @@ static void bdrv_close(BlockDriverState *bs)
 {
     BdrvAioNotifier *ban, *ban_next;
 
-    if (bs->job) {
-        block_job_cancel_sync(bs->job);
-    }
+    assert(!bs->job);
 
     /* Disable I/O limits and drain all pending throttled requests */
     if (bs->io_limits_enabled) {
@@ -1971,13 +1969,44 @@ static void bdrv_close(BlockDriverState *bs)
 void bdrv_close_all(void)
 {
     BlockDriverState *bs;
+    AioContext *aio_context;
+    int original_refcount = 0;
 
-    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
-        AioContext *aio_context = bdrv_get_aio_context(bs);
+    /* Drop references from requests still in flight, such as canceled block
+     * jobs whose AIO context has not been polled yet */
+    bdrv_drain_all();
 
-        aio_context_acquire(aio_context);
-        bdrv_close(bs);
-        aio_context_release(aio_context);
+    blockdev_close_all_bdrv_states();
+    blk_remove_all_bs();
+
+    /* Cancel all block jobs */
+    while (!QTAILQ_EMPTY(&all_bdrv_states)) {
+        QTAILQ_FOREACH(bs, &all_bdrv_states, bs_list) {
+            aio_context = bdrv_get_aio_context(bs);
+
+            aio_context_acquire(aio_context);
+            if (bs->job) {
+                /* So we can safely query the current refcount */
+                bdrv_ref(bs);
+                original_refcount = bs->refcnt;
+
+                block_job_cancel_sync(bs->job);
+                aio_context_release(aio_context);
+                break;
+            }
+            aio_context_release(aio_context);
+        }
+
+        /* All the remaining BlockDriverStates are referenced directly or
+         * indirectly from block jobs, so there needs to be at least one BDS
+         * directly used by a block job */
+        assert(bs);
+
+        /* Wait for the block job to release its reference */
+        while (bs->refcnt >= original_refcount) {
+            aio_poll(aio_context, true);
+        }
+        bdrv_unref(bs);
     }
 }
 
-- 
2.6.2