From: Emanuele Giuseppe Esposito <eesposit@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
Stefan Hajnoczi <stefanha@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>, John Snow <jsnow@redhat.com>
Subject: Re: [PATCH v3 2/5] block-copy: improve comments of BlockCopyTask and BlockCopyState types and functions
Date: Thu, 10 Jun 2021 12:14:23 +0200 [thread overview]
Message-ID: <ff0e4ff0-681c-2827-31cd-efc6b01360ef@redhat.com> (raw)
In-Reply-To: <b466cc82-3905-acba-a2f8-e0b3cbbe2af1@virtuozzo.com>
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?
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
next prev parent reply other threads:[~2021-06-10 10:41 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-08 7:33 [PATCH v3 0/5] block-copy: protect block-copy internal structures Emanuele Giuseppe Esposito
2021-06-08 7:33 ` [PATCH v3 1/5] block-copy: streamline choice of copy_range vs. read/write Emanuele Giuseppe Esposito
2021-06-09 8:51 ` Vladimir Sementsov-Ogievskiy
2021-06-09 9:33 ` Paolo Bonzini
2021-06-09 10:09 ` Vladimir Sementsov-Ogievskiy
2021-06-09 10:54 ` Vladimir Sementsov-Ogievskiy
2021-06-08 7:33 ` [PATCH v3 2/5] block-copy: improve comments of BlockCopyTask and BlockCopyState types and functions Emanuele Giuseppe Esposito
2021-06-09 9:12 ` Vladimir Sementsov-Ogievskiy
2021-06-10 10:14 ` Emanuele Giuseppe Esposito [this message]
2021-06-10 10:27 ` Vladimir Sementsov-Ogievskiy
2021-06-10 10:46 ` Emanuele Giuseppe Esposito
2021-06-10 11:12 ` Vladimir Sementsov-Ogievskiy
2021-06-10 14:21 ` Emanuele Giuseppe Esposito
2021-06-10 15:05 ` Vladimir Sementsov-Ogievskiy
2021-06-08 7:33 ` [PATCH v3 3/5] block-copy: move progress_set_remaining in block_copy_task_end Emanuele Giuseppe Esposito
2021-06-08 7:33 ` [PATCH v3 4/5] block-copy: add a CoMutex Emanuele Giuseppe Esposito
2021-06-09 12:25 ` Vladimir Sementsov-Ogievskiy
2021-06-10 14:49 ` Emanuele Giuseppe Esposito
2021-06-08 7:33 ` [PATCH v3 5/5] block-copy: atomic .cancelled and .finished fields in BlockCopyCallState Emanuele Giuseppe Esposito
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