qcow2-threads: qcow2_co_do_compress: protect queuing by mutex

Drop dependence on AioContext lock.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190506142741.41731-5-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/qcow2-threads.c

@@ -158,15 +158,19 @@ qcow2_co_do_compress(BlockDriverState *bs, void *dest, size_t dest_size,
         .func = func,
     };
 
+    qemu_co_mutex_lock(&s->lock);
     while (s->nb_compress_threads >= MAX_COMPRESS_THREADS) {
-        qemu_co_queue_wait(&s->compress_wait_queue, NULL);
+        qemu_co_queue_wait(&s->compress_wait_queue, &s->lock);
     }
-
     s->nb_compress_threads++;
+    qemu_co_mutex_unlock(&s->lock);
+
     thread_pool_submit_co(pool, qcow2_compress_pool_func, &arg);
-    s->nb_compress_threads--;
 
+    qemu_co_mutex_lock(&s->lock);
+    s->nb_compress_threads--;
     qemu_co_queue_next(&s->compress_wait_queue);
+    qemu_co_mutex_unlock(&s->lock);
 
     return arg.ret;
 }