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| From: | Emanuele Giuseppe Esposito |
| Subject: | Re: [PATCH v3 2/5] block-copy: improve comments of BlockCopyTask and BlockCopyState types and functions |
| Date: | Thu, 10 Jun 2021 16:21:11 +0200 |
| User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 |
On 10/06/2021 13:12, Vladimir Sementsov-Ogievskiy wrote:
10.06.2021 13:46, Emanuele Giuseppe Esposito wrote:On 10/06/2021 12:27, Vladimir Sementsov-Ogievskiy wrote:10.06.2021 13:14, Emanuele Giuseppe Esposito wrote: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 <eesposit@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?yesHere 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.But we shrink task only once, and we do it under mutex, so we are OK I think?I think you understood, but just in case: I am thinking the case where we have:
But maybe I am overcomplicating.Both shrink and find_ are done under mutex, so they can't intersect. But yes, we should keep in mind that if we do find_ under mutex, and then release mutex, the information get from find_ may become incorrect.Check callers of find_conflicting_task_locked(): block_copy_wait_one has one critical section.if no conflicting tasks we are OK.. Are we? Ok, look at the only caller of block_copy_wait_one() - block_copy_common().assume block_copy_dirty_clusters() returns 0, so there no dirty bits at some moment...than in parallel thread some task may finish with failure, leaving some new dirty bits.. Then we check that there no conflicting tasks.. And then we go out of the loop, when actually we must retry for these new dirty bits.So I'm afraid you are right, we are not threadsafe yet in block_copy_common(), as we should check conflicting tasks and dirty bits in same critical section to be consistent.
Wait, we are talking about two different problems:- What I wanted to point out has to do with @bytes, not (as far as I understand) with the dirty bits. From the example I made below, I assume there are 3 separate non-overlapping critical sections:
T1: block_copy_task_create() T2: find_conflicting_tasks() <-- sees the initial taskT1: task_shrink() <-- bytes are updated, T2 saw the wrong amount of bytes. This might or might not have consequences, I am not sure.
T1 creates the task, T2 iterates to search for conflicting tasks (called from a parallel block_copy_wait_one), T1 shrinks the current task. I think that T2 in this case misses the updated task, even though the worst it can happen is that the task is smaller, so a false positive (a task is not conflicting but might be marked as conflicting). The outcome is that T2 is waiting for a task it shouldn't, but there is no error there.
- Your point is about a task failing between block_copy_dirty_clusters and block_copy_wait_one. The task failing calls block_copy_task_end and sets the dirty bitmap, but at that point block_copy_wait_one won't check it anymore and the bitmap is left dirty. I think the default behavior here should be that block_copy_dirty_clusters() is called and a new task is created. This, as you pointed out, is a proper error.
In this case, we need to stop iterating only when 1) the whole bitmap is clear, and 2) no conflicting task is present.
Therefore a possible solution can be the one below:
int stop_looping = 0;
...
do {
// create all the tasks, clears the bitmap but
// adds tasks to the task list
block_copy_dirty_clusters();
/* here a task can fail, but then the dirty map will be set */
lock();
// make sure no task is running for this operation
stop_looping = (find_conflicting_task() == NULL);
// make sure that the dirty bitmap is clear
stop_looping |= (!bdrv_dirty_bitmap_next_dirty_area()) << 1;
unlock();
/* if stop_looping is == 0, no task can fail */
/* if a task fails here, the if below won't see it but it will
* block_copy_dirty_clusters in the next iteration */
if (stop_looping & 1) {
// there is some conflicting task, wait for it
qemu_co_queue_wait(&task->wait_queue);
}
} while(stop_looping != 0);
...
What do you think?
Emanuele
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.This way find_conflicting_tasks will just skip our new creating task.. And we'll get conflict when try to add our new task. No, we should add task to the list at same critical section where we clear dirty bits from the bitmap.I agree, with the above.So to me the most correct solution would be to call create and shrink in the same lock, but this creates a much wider critical section.Alternatively, I can leave it as it is and just update the comment.Then we shrink task in another critical section, it should be OK too.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.On last review it seemed to me that you actually protect bytes by critical section where it is needed. So here I'm saying only about the comment..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.find_conflicting_tasks only reads bytes, so it can't make damage.. Anyway making critical sections a bit wider won't hurt.
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