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 <eespo...@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
CoMutex)?
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);
qemu_co_mutex_unlock(&s->tasks_lock);
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,
task->bytes);
}
qemu_co_mutex_lock(&task->s->tasks_lock);
+ co_put_to_shres(task->s->mem, task->bytes);
task->s->in_flight_bytes -= task->bytes;
QLIST_REMOVE(task, list);
progress_set_remaining(task->s->progress,
@@ -379,7 +381,6 @@ static coroutine_fn int
block_copy_task_run(AioTaskPool *pool,
aio_task_pool_wait_slot(pool);
if (aio_task_pool_status(pool) < 0) {
- co_put_to_shres(task->s->mem, task->bytes);
block_copy_task_end(task, -ECANCELED);
g_free(task);
return -ECANCELED;
@@ -498,7 +499,6 @@ static coroutine_fn int
block_copy_task_entry(AioTask *task)
}
qemu_mutex_unlock(&t->s->calls_lock);
- 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);
