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Re: [PATCH 5/6] co-shared-resource: protect with a mutex

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH 5/6] co-shared-resource: protect with a mutex
Date: Sat, 15 May 2021 00:15:29 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1

14.05.2021 20:28, Emanuele Giuseppe Esposito wrote:

On 14/05/2021 17:30, Vladimir Sementsov-Ogievskiy wrote:
14.05.2021 17:32, Emanuele Giuseppe Esposito wrote:

On 14/05/2021 16:26, Vladimir Sementsov-Ogievskiy wrote:
14.05.2021 17:10, Emanuele Giuseppe Esposito wrote:

On 12/05/2021 17:44, Stefan Hajnoczi wrote:
On Mon, May 10, 2021 at 10:59:40AM +0200, Emanuele Giuseppe Esposito wrote:
co-shared-resource is currently not thread-safe, as also reported
in co-shared-resource.h. Add a QemuMutex because co_try_get_from_shres
can also be invoked from non-coroutine context.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
  util/qemu-co-shared-resource.c | 26 ++++++++++++++++++++++----
  1 file changed, 22 insertions(+), 4 deletions(-)

Hmm...this thread-safety change is more fine-grained than I was
expecting. If we follow this strategy basically any data structure used
by coroutines needs its own fine-grained lock (like Java's Object base
class which has its own lock).

I'm not sure I like it since callers may still need coarser grained
locks to protect their own state or synchronize access to multiple
items of data. Also, some callers may not need thread-safety.

Can the caller to be responsible for locking instead (e.g. using

Right now co-shared-resource is being used only by block-copy, so I guess 
locking it from the caller or within the API won't really matter in this case.

One possible idea on how to delegate this to the caller without adding 
additional small lock/unlock in block-copy is to move co_get_from_shres in 
block_copy_task_end, and calling it only when a boolean passed to 
block_copy_task_end is true.

Otherwise make b_c_task_end always call co_get_from_shres and then include 
co_get_from_shres in block_copy_task_create, so that we always add and in case 
remove (if error) in the shared resource.

Something like:

diff --git a/block/block-copy.c b/block/block-copy.c
index 3a447a7c3d..1e4914b0cb 100644
--- a/block/block-copy.c
+++ b/block/block-copy.c
@@ -233,6 +233,7 @@ static coroutine_fn BlockCopyTask 
*block_copy_task_create(BlockCopyState *s,
      /* region is dirty, so no existent tasks possible in it */
      assert(!find_conflicting_task(s, offset, bytes));
      QLIST_INSERT_HEAD(&s->tasks, task, list);
+    co_get_from_shres(s->mem, task->bytes);

      return task;
@@ -269,6 +270,7 @@ static void coroutine_fn block_copy_task_end(BlockCopyTask 
*task, int ret)
          bdrv_set_dirty_bitmap(task->s->copy_bitmap, task->offset, 
+    co_put_to_shres(task->s->mem, task->bytes);
      task->s->in_flight_bytes -= task->bytes;
      QLIST_REMOVE(task, list);
@@ -379,7 +381,6 @@ static coroutine_fn int block_copy_task_run(AioTaskPool 

      if (aio_task_pool_status(pool) < 0) {
-        co_put_to_shres(task->s->mem, task->bytes);
          block_copy_task_end(task, -ECANCELED);
          return -ECANCELED;
@@ -498,7 +499,6 @@ static coroutine_fn int block_copy_task_entry(AioTask *task)

-    co_put_to_shres(t->s->mem, t->bytes);
      block_copy_task_end(t, ret);

      return ret;
@@ -687,8 +687,6 @@ block_copy_dirty_clusters(BlockCopyCallState *call_state)

          trace_block_copy_process(s, task->offset);

-        co_get_from_shres(s->mem, task->bytes);

we want to get from shres here, after possible call to block_copy_task_shrink(), 
as task->bytes may be reduced.

Ah right, I missed that. So I guess if we want the caller to protect 
co-shared-resource, get_from_shres stays where it is, and put_ instead can 
still go into task_end (with a boolean enabling it).

honestly, I don't follow how it helps thread-safety

 From my understanding, the whole point here is to have no lock in 
co-shared-resource but let the caller take care of it (block-copy).

The above was just an idea on how to do it.

But how moving co_put_to_shres() make it thread-safe? Nothing in block-copy is 
thread-safe yet..

Best regards,

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