On 09/06/2021 11:12, Vladimir Sementsov-Ogievskiy wrote:
08.06.2021 10:33, Emanuele Giuseppe Esposito wrote:
As done in BlockCopyCallState, categorize BlockCopyTask
and BlockCopyState in IN, State and OUT fields.
This is just to understand which field has to be protected with a lock.
.sleep_state is handled in the series "coroutine: new sleep/wake API"
and thus here left as TODO.
Signed-off-by: Emanuele Giuseppe Esposito <eespo...@redhat.com>
---
block/block-copy.c | 47 ++++++++++++++++++++++++++++++----------------
1 file changed, 31 insertions(+), 16 deletions(-)
diff --git a/block/block-copy.c b/block/block-copy.c
index d58051288b..b3533a3003 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -56,25 +56,33 @@ typedef struct BlockCopyCallState {
QLIST_ENTRY(BlockCopyCallState) list;
/* State */
Why previous @list field is not in the state? For sure it's not an IN
parameter and should be protected somehow.
- int ret;
bool finished;
- QemuCoSleep sleep;
- bool cancelled;
+ QemuCoSleep sleep; /* TODO: protect API with a lock */
/* OUT parameters */
+ bool cancelled;
bool error_is_read;
+ int ret;
} BlockCopyCallState;
typedef struct BlockCopyTask {
AioTask task;
+ /*
+ * IN parameters. Initialized in block_copy_task_create()
+ * and never changed.
+ */
BlockCopyState *s;
BlockCopyCallState *call_state;
int64_t offset;
- int64_t bytes;
- BlockCopyMethod method;
- QLIST_ENTRY(BlockCopyTask) list;
+ int64_t bytes; /* only re-set in task_shrink, before running the
task */
+ BlockCopyMethod method; /* initialized in
block_copy_dirty_clusters() */
hmm. to be precise method is initialized in block_copy_task_create.
And after block_copy_task_create finished, task is in the list and can
be read by parallel block_copy_dirty_clusters(). So, @bytes is part of
State, we must protect it..
So if I understand correctly, you refer to the fact that a parallel
block_copy_dirty_clusters() can create another task and search with
find_conflicting_task_locked(), or in general also block_copy_wait_one()
can do the same in parallel, correct?
Here there is also another problem: if we add the task to the list and
then shrink it in two different critical sections, we are going to have
problems because in the meanwhile find_conflicting_tasks can be issued
in parallel.
So, is there a reason why we don't want
QLIST_INSERT_HEAD(&s->tasks, task, list);
in block_copy_dirty_clusters()?
By doing that, I think we also spare @bytes from the critical section,
since it is only read from that point onwards.
I am also trying to see if I can group some critical sections.
Btw I think we already talked about @bytes and it's not the first time
we switch it from IN to STATE and vice-versa...
I mean, I agree with you but it starts to be confusing.
This also goes against your comment later in patch 4,
@@ -212,7 +222,7 @@ static BlockCopyTask *block_copy_task_create(BlockCopyState
*s,
bytes = QEMU_ALIGN_UP(bytes, s->cluster_size);
/* region is dirty, so no existent tasks possible in it */
- assert(!find_conflicting_task(s, offset, bytes));
+ assert(!find_conflicting_task_locked(s, offset, bytes));
bdrv_reset_dirty_bitmap(s->copy_bitmap, offset, bytes);
s->in_flight_bytes += bytes;
@@ -248,16 +258,19 @@ static void coroutine_fn
block_copy_task_shrink(BlockCopyTask *task,
The function reads task->bytes not under mutex.. It's safe, as only that function is modifying the field, and it's called once. Still, let's make critical section a little bit wider, just for simplicity. I mean, simple QEMU_LOCK_GUARD() at start of function.
Where if I understand correctly, it is not safe, because
find_conflicting_tasks might search the non-updated task.
Thank you,
Emanuele
