[pve-devel] applied: [PATCH qemu 1/3] alloc track: acquire BS AIO context during dropping

Thomas Lamprecht t.lamprecht at proxmox.com
Tue Apr 6 16:41:57 CEST 2021

ran into this when live-restoring a backup configured for IO-threads,
got the good ol':
> qemu: qemu_mutex_unlock_impl: Operation not permitted

Checking out the history of the related bdrv_backup_top_drop(*bs)
method, we can see that it used to do the AIO context acquiring too,
but in the backup path this was problematic and was changed to be
higher up in the call path in a upstream series from Stefan[0].

That said, this is a completely different code path and it is safe to
do so here. We always run from the main threads's AIO context here
and we call it only indirectly once, guarded by checking for
`s->drop_state == DropNone` and set `s->drop_state = DropRequested`
shortly before we schedule the track_drop() in a bh.

[0]: https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg09139.html

Signed-off-by: Thomas Lamprecht <t.lamprecht at proxmox.com>

It's for an opt-in, not yet exposed feature, so not much to break here (also it
was clearly broken, and this _is_ the right fix IMO, if anything one could
argue if here is the right place, but checking callers and testing various
scenarios it seems right)

 debian/patches/pve/0047-block-add-alloc-track-driver.patch | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/debian/patches/pve/0047-block-add-alloc-track-driver.patch b/debian/patches/pve/0047-block-add-alloc-track-driver.patch
index 232ad7e..8eed42f 100644
--- a/debian/patches/pve/0047-block-add-alloc-track-driver.patch
+++ b/debian/patches/pve/0047-block-add-alloc-track-driver.patch
@@ -34,7 +34,7 @@ new file mode 100644
 index 0000000000..b579380279
 --- /dev/null
 +++ b/block/alloc-track.c
-@@ -0,0 +1,342 @@
+@@ -0,0 +1,345 @@
 + * Node to allow backing images to be applied to any node. Assumes a blank
 + * image to begin with, only new writes are tracked as allocated, thus this
@@ -313,6 +313,8 @@ index 0000000000..b579380279
 +        aio_bh_schedule_oneshot(qemu_get_aio_context(), track_drop, opaque);
 +        return;
 +    }
++    AioContext *aio_context = bdrv_get_aio_context(bs);
++    aio_context_acquire(aio_context);
 +    bdrv_drained_begin(bs);
@@ -324,6 +326,7 @@ index 0000000000..b579380279
 +    bdrv_set_backing_hd(bs, NULL, &error_abort);
 +    bdrv_drained_end(bs);
 +    bdrv_unref(bs);
++    aio_context_release(aio_context);
 +static int track_change_backing_file(BlockDriverState *bs,

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